pixel-formats: add RGBA bits and type fields

pixel-formats: add RGBA bits and type fields

These fields are necessary when looking for an EGLConfig matching a pixel
format, but the configs do not expose a native visual id. Such happens on the
EGL surfaceless platform where one does not actually care about the exact pixel
format, one just cares it has the right number of bits for each channel and the
right component type.

FP16 formats are coming, so this paves way for them too, allowing them to be
described.

The FIXED/FLOAT terminology comes from EGL_EXT_pixel_format_float.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>