only supported ones are: RGBA/BGRA/rgbA/bgrA (decoder)
as well as: WebPPictureImportRGB/RGBX/RGBA (encoder).
(note: extras/get_disto is affected too)
Change-Id: If6c4f95054ca15759c4e289fb3b4c352b3521c2c
(cherry picked from commit 6de20df02c63b88f408f4b6f52830db339dfdd04)