mkv: Write SimpleBlock instead of Block for frame headers

mkvtoolnix writes these by default since 2009/04/13.
Slightly simplifies muxer and allows 'mkvinfo -s' to show B-frames
as 'B' (but not B-ref frames).
mkv: Write the x264 version into the file header

This only updates the "writing application"; matroska_ebml.c is the
"muxing application", but the version string for that is still hardcoded.
Mark cli_input/output_t variables as const when possible
Make the ABR buffer consider the distance to the end of the video
Should improve bitrate accuracy in 2-pass mode.
May also slightly improve quality by allowing more variation earlier-on in a file.

Also fix abr_buffer with 1-pass: it does something very different than what it does for 2-pass.
Thus, the earlier change that increased it based on threads caused 1-pass ABR to be somewhat less accurate.
Use #ifdef instead of #if in checkasm
ARM NEON versions of weightp functions
iPhone compilation support
Also add --sysroot to configure options

To build for iPhone 3gs / iPod touch 3g:
CC=/Developer/Platforms/iPhoneOS.platform/Developer/usr/bin/gcc ./configure --host=arm-apple-darwin --sysroot=/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS3.0.sdk

For older devices, add
--extra-cflags='-arch armv6 -mcpu=arm1176jzf-s' --extra-ldflags='-arch armv6' --disable-asm
Don't even try direct temporal when it would give junk MVs
In PbBbP pyramid structure, the last "b" cannot use temporal because L0Ref0(L1Ref0) != L0Ref0.
Don't even bother analyzing it, just use spatial.
Should improve speed and direct auto effectiveness in CRF and 1-pass modes when b-pyramid is used.
Also makes --direct temporal useful with --b-pyramid, since it will fall back to spatial for frames where temporal is broken.
Remove unnecessary PIC support macros
yasm has a directive to enable PIC globally
Fix compilation on ARM
Fix I and B-frame QPs with threads
Rounding errors resulted in slightly wrong QPs with threads enabled.
Fix integer overflow in chroma SSD check
Could cause bad skips at very high quantizers on extreme inputs.
Fix overread of scratch buffer
Could cause crashes on non-mod16 frames.
Fix stupid regression in interlaced in r1430
With ref > 8 or b-pyramid, an array over-read could cause slightly incorrect B-frames.
Add GPAC version check
SimpleBlock requires Matroska Doctype v2
Much faster and simpler direct spatial calculation
Keep track of macroblock partitions
Allows vastly simpler motion compensation and direct MV calculation.
Add temporal predictor support to interlaced encoding
0.5-1% better compression in interlaced mode
Much faster and more efficient MVD handling
Store MV deltas as clipped absolute values.
This means CABAC no longer has to calculate absolute values in MV context selection.
This also lets us cut the memory spent on MVDs by a factor of 2, speeding up cache_mvd and reducing memory usage by 32*threads*(num macroblocks) bytes.
On a Core i7 encoding 1080p, this is about 3 megabytes saved.