adds a generic examples/image_dec.[ch] entry point too.
WebPGuessImageType() can be used to infer image type.
Change-Id: I8337e7b6ad91863c9cf118e4791668d2d175079b
The goal is not to replace our autotools configuration, but to
provide a minimal CMake file that can build the lib, cwebp and
dwebp.
Change-Id: I3be343bd698d118c5f00172449d232d87e868f23