Quite handy and simple (for now).
It's not yet incorporated within the build system (autotools or
makefile.unix), because it gets complicated with OpenGL. TODO(later).
For now, there's some ready-to-use command line for compiling on Linux
or Mac in the header of vwebp.c
Later, this tool will be supplemented with support for animation,
tiling, zooming, etc.
Change-Id: I292972cea4862536afbe8c9ec444c590d152f086
