[truetype] Fix positioning of composite glyphs.
Problem reported by Nigel Tao <nigeltao@golang.org>.

* src/truetype/ttgload.c (TT_Hint_Glyph): Remove code that shifts
the glyph (component) by a fractional value computed from the LSB
phantom point.  This is wrong, since the horizontal phantom points
get rounded horizontally later on.
Formatting.
[truetype] Fix scaling of vertical phantom points.

* src/truetype/ttgload.c (load_truetype_glyph): Scale pp3.x and
pp4.x also.
[autofit] s/AF_SCRIPT_NONE/AF_SCRIPT_UNASSIGNED/.
[autofit] s/DFLT/NONE/, s/dflt/none/.
[documentation] Add section how to include FreeType header files.
Problem reported by David Kastrup <dak@gnu.org>.

Surprisingly, a description how to do that was completely missing in
the API reference.

* include/freetype.h, include/ftchapters.h: New documentation
section `header_inclusion'.
[truetype] Simplify logic of rendering modes.

This patch unifies the subpixel and non-subpixel cases.

* src/truetype/ttinterp.h (TT_ExecContextRec): Remove
`grayscale_hinting'; all code should refer to `grayscale' instead.
Remove unused `native_hinting' member.
Rename `subpixel_hinting' member to `subpixel.

* src/truetype/ttgload.c (TT_LOADER_SET_PP): Updated.
(tt_loader_init): Updated.

* src/truetype/ttinterp.c (Ins_GETINFO): Simplify.
Updated.
[autofit] s/AF_Script_/AF_WritingSystem_/ where appropriate.