tiffdec: add grayscale support
tiffdec: add grayscale support

the file is always decoded to RGBA, so just the samples_per_px needed an
update

Bug: webp:563
Change-Id: I95d95c3b9e45dc8ecb2223f89f4ba791f0b21e8b

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp