vp9/x86: intra prediction SIMD.

Partially based on h264_intrapred. (I hope to eventually merge these
two intrapred implementations back together.)
vp9: split decode_coeff_b loop inside txsz branch.

The advantage of this is that the is32x32 division branch in
decode_coeffs_b is removed from the inner loop to outside the block
coef decoding loop in decode_coeffs. Also, it allows us to merge the
txsz branches from the block coef decoding loop, the context merge
and the context split.
doc/examples: remove pathes from doxy examples

This makes the examples page less cluttered

Reviewed-by: Clément Bœsch <u@pkh.me>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
tta/x86: add ff_ttafilter_process_dec_{ssse3, sse4}

Results are from a Win64 build running on an AMD FX 6300

1121 decicycles in ttafilter_process_dec_c, 16777112 runs, 104 skips
522 decicycles in ff_ttafilter_process_dec_ssse3, 16777149 runs, 67 skips
477 decicycles in ff_ttafilter_process_dec_sse4, 16777156 runs, 60 skips

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>