This is to infer the needed conversion to YUV(A) or RGB(A).
This is useful to avoid some conversion steps between ARGB and YUVA.
For instance, if the input file is a JPEG, we decode to RGB and
convert to YUV right away, without the intermediate step to ARGB.
The only caveat is that cropping/scaling might give slightly different result,
because of YUV420 downsampling. Therefore, we omit this feature
at cwebp level, when -crop or -rescale is used.
Change-Id: I5a3abe5108982f2a4570e841e3d9baffc73f5bee
