[RFC] WebP image format support addition

WebP is an image format based on VP8 standard. I have implemented WebP decoder support for OpenCV. Tested decoding with one of the provided samples (samples/c/morphology.c code) and WebP image is getting decoded successfully. In coming weeks I will start working on WebP encoder.

This is just a request for comment and not a pull request as the code quality is not yet in par with OpenCV standards. It will be great if I could get comments at an early stage. I welcome any sort of comment so that the code quality can be improved and brought up to OpenCV standards.