Speed up ARMv7 support.

When building for ARMv7 with thumb2 instructions, the optimized
`FT_MulFix_arm' assembly routine was not being used.

The reason for this is in the `ftconfig.h' header, namely:

- The assembly routine uses the `smull' instruction which is not
  available when generating Thumb-1 machine code.  It is available
  in Thumb-2 mode, though.

- The header was written a long time ago before Thumb-2 became
  widely popular (e.g. with Android).  So it simply doesn't use the
  assembly routine if the `__thumb__' built-in macro is defined.

- When compiling in Thumb-2 mode, the compiler will define both
  `__thumb__' and `__thumb2__'.

By checking for `(__thumb2__ || !__thumb__)', we ensure that the
assembly routine is only avoided when generating Thumb-1 code.

Given that this is performance-sensitive function, this improves
`ftbench' as follows on a Galaxy Nexus:

                           Before (us/op)   After (us/op)

  - loading Arial.ttf glyphs at 14 ppem [1]

      Load                   34.285          33.098

  - same operation with the light auto-hinter [2]

      Load                   31.317          29.590

  - same operation without hinting [3]

      Load                    6.143           5.376

  - loading Arial.ttf advances at 14 ppem [4]

      Load_Advances (normal) 34.216          33.016
      Load_Advances (fast)    0.176           0.176

  [1] ftbench -t 5 -p -s 14 -b a -f 0008 Arial.ttf
  [2] ftbench -t 5 -p -s 14 -b a -r 1 -f 0028 Arial.ttf
  [3] ftbench -t 5 -p -s 14 -b a -f 000a Arial.ttf
  [4] ftbench -t 5 -p -s 14 -b b -f 0008 Arial.ttf

* builds/unix/ftconfig.in, include/freetype/config/ftconfig.h
(FT_MULFIX_ASSEMBLER): Fix handling for ARMv7.
Optimize FT_MulFix for x86_64 GCC builds.

This patch provides an optimized `FT_MulFix' implementation for
x86_64 machines when FreeType is built with GCC, or compatible
compilers like Clang.

Example:
  bin/ftbench -p -t 5 -s 14 -f 0008 Arial.ttf

Before:

  Load                       4.863 us/op
  Load_Advances (Normal)     4.816 us/op
  Load_Advances (Fast)       0.028 us/op
  Render                     2.753 us/op
  Get_Glyph                  0.463 us/op
  Get_CBox                   0.077 us/op
  Get_Char_Index             0.023 us/op
  Iterate CMap              13.898 us/op
  New_Face                  12.368 us/op
  Embolden                   0.028 us/op
  Get_BBox                   0.302 us/op

After:

  Load                       4.617 us/op
  Load_Advances (Normal)     4.645 us/op
  Load_Advances (Fast)       0.027 us/op
  Render                     2.789 us/op
  Get_Glyph                  0.460 us/op
  Get_CBox                   0.077 us/op
  Get_Char_Index             0.024 us/op
  Iterate CMap              13.403 us/op
  New_Face                  12.278 us/op
  Embolden                   0.028 us/op
  Get_BBox                   0.301 us/op

* builds/unix/ftconfig.in, include/freetype/config/ftconfig.h
(FT_MulFix_x86_64): New function.
Add assembler code for TT_MulFix14 and TT_DotFix14.

This patch provides slightly optimized versions for ARM, x86, and
x86_64 CPUs if built with GCC.

Also remove some dead code.

* src/truetype/ttinterp.c (TT_MulFix14_arm, TT_MulFix14_long_long,
TT_DotFix14_long_long): New functions.
[smooth] Improve performance.

Provide a work-around for an ARM-specific performance bug in GCC.
This speeds up the rasterizer by more than 5%.

Also slightly optimize `set_gray_cell' and `gray_record_cell' (which
also improves performance on other platforms by a tiny bit (<1%).

* src/smooth/ftgrays.c (FT_DIV_MOD): New macro.
Use it where appropriate.

(gray_record_cell, gray_set_cell, gray_move_to,
gray_convert_glyph_inner): Streamline condition handling.
[smooth] Fix segfault caused by previous commit.

* src/smooth/ftgrays.c (gray_set_cell): Always compute
`ras.invalid'.
[sfnt] Ignore invalid magic number in `head' or `bhed'.

Other font engines seem to ignore it also.  Problem reported by
Hin-Tak Leung <htl10@users.sourceforge.net>.

* src/sfnt/ttload.c (check_table_dir): Don't abort but warn only if
we have an invalid magic number.
[sfnt] Add support for Apple's `sbix' color bitmap table.

* include/freetype/internal/tttypes.h (TT_SBit_MetricsRec): Widen
fields to FT_Short and FT_UShort, respectively.
(TT_SBitTableType): New enumeration.
(TT_FaceRec): Add `sbit_table_type' field.

* include/freetype/tttags.h (TTAG_sbix): New macro.

* src/sfnt/pngshim.c (Load_SBit_Png): Pass a more generic
FT_GlyphSlot argument instead FT_Bitmap.
Add flag to control map and metrics handling.
Update all users.

* src/sfnt/ttsbit.c: Include `ttmtx.h'.
(tt_face_load_eblc): Renamed to...
(tt_face_load_sbit): This.
Handlic `sbix' bitmaps.
(tt_face_free_eblc): Renamed to...
(tt_face_load_sbit): This.
Updated.
(tt_face_load_strike_metrics): Handle `sbix' bitmaps.
(tt_face_load_sbix_image): New function.
(tt_sbit_decoder_alloc_bitmap, tt_sbit_decoder_load_image,
tt_sbit_decoder_load_byte_aligned, tt_sbit_decoder_load_bit_aligned,
tt_sbit_decoder_load_compound, tt_sbit_decoder_load_png,
tt_sbit_decoder_load_image, tt_sbit_decoder_load_bitmap): Don't pass
and handle load flags.
(tt_sbit_decoder_load_bitmap) [!FT_CONFIG_OPTION_USE_PNG]: Better
handle formats 17-19.
Move color to grayscale conversion to...
(tt_face_load_sbit_image): Here.
Handle `sbix' bitmaps.

* src/sfnt/pngshim.h: Updated.
* src/sfnt/ttsbit.h: Updated.
* src/sfnt/sfdriver.c: Updated.
[sfnt] `sbix' fix-ups.

* src/sfnt/sfobjs.c (sfnt_load_face): Apple's `sbix' color bitmaps
are rendered scaled and then the `glyf' outline rendered on top.  We
don't support that yet, so just ignore the `glyf' outline and
advertise it as a bitmap-only font.

* src/sfnt/ttsbit.c (tt_face_load_strike_metrics)
[TT_SBIT_TABLE_TYPE_SBIX]: Return metrics in 26.6 units.
(tt_face_load_sbix_image): Typo.
[autofit] Fix segment classification for blue zones.

The old code (essentially unchanged since the very beginning)
incorrectly handled this configuration

               x -o- x
                /   \
               /     \
              /       \
             o         o

as flat and this

                o               o
               /               /
             x|              x|
              |               |
              o---------------o

as round.  (`o' and `x' are on and off points, respectively).

This is a major change which should improve the rendering results
enormously for many TrueType fonts, especially in the range approx.
20-40ppem, fixing the appearance of many overshoots.

* src/autofit/aflatin.c (af_latin_metrics_init_blues): Look at the
first and last points of the segment, not the points right before
and after.
[sfnt] Fix `sbix' table version handling.

* src/sfnt/ttsbit.c (tt_face_load_sbit) [TT_SBIT_TABLE_TYPE_SBIX]:
USHORT version numbers are to be considered as `minor'.
* docs/INSTALL.GNU: Updated.
Merge branch 'master' of git://git.sv.nongnu.org/freetype/freetype2