cogl_read_pixels: Always use GL_RGBA/GL_UNSIGNED_BYTE under GLES

cogl_read_pixels: Always use GL_RGBA/GL_UNSIGNED_BYTE under GLES

Under GLES glReadPixels is documented to only support GL_RGBA with
GL_UNSIGNED_BYTE and an implementation specfic format which can be
fetched with glGet, GL_IMPLEMENTATION_COLOR_READ_FORMAT_OES and
GL_IMPLEMENTATION_COLOR_READ_TYPE_OES. This patch makes it always read
using GL_RGBA and GL_UNSIGNED_BYTE and then convert the results if
neccessary.

This has some room for improvement because it doesn't attempt to use
the implementation specific format. Also the conversion is somewhat
wasteful because there are currently no cogl_bitmap_* functions to
convert without allocating a new buffer so it ends up doing an
intermediate copy.

http://bugzilla.openedhand.com/show_bug.cgi?id=2057