we map the input file into memory, even in the non-stdin case.
This is less efficient than letting the png/jpeg/... decoding libraries
use fread()'s, but more general.
Change-Id: I4501cb9a1daf69593eb8e3326c115cd8cbdf92fd
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
