This patch is adding support for google webp image format, if you
don't know it a quick recap is to say that according to their tests
they claim it is better than png and jpeg.
Follow the link below for some more details:
https://developers.google.com/speed/webp/
Signed-off-by: Carlos R. Mafra <crmafra@gmail.com>
An enum is always a better idea as it allows the compiler to do
some checks, and as this info is internal only to the WRLib it
will not change the API.
The functions are declared in different files but they were called
in another file which re-declared the prototypes. This is dangerous
as it can lead to misaligned prototypes when functions changes.
They are now grouped in the library internal header 'imgformat.h'
