swscale: prevent an invalid write of 1 byte

Bug-Id: 772
CC: libav-stable@libav.org
Found-By: Justin Ruggles <justin.ruggles@gmail.com>
pixdesc: Support pixelformat aliases

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
avutil: add alias names for gray 8/16 colour spaces
avutil: rename AV_PIX_FMT_Y400A to AV_PIX_FMT_YA8

The rationale is that you have a packed format in form
<greyscale sample> <alpha sample> <greyscale sample> <alpha sample>
and shortening greyscale to 'G' might make one thing about Greenscale instead.
An alias pixel format and color space name are provided for compatibility.
avutil: add AV_PIX_FMT_YA16 pixel format
swscale: support AV_PIX_FMT_YA16 as input
png: disable broken MMX/SIMD code for bpp <= 2

The decoder was producing different results when ASM was disabled.
Based on a long debug session with Kostya.
png: support reading gray+alpha at 16 bits
tiff: support reading gray+alpha at 16 bits
tiff: support reading gray+alpha at 8 bits

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
fate: png-suite

Every supported format is converted to RGB.