swscale: support AV_PIX_FMT_YA16 as input
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 RGBA.