picking: Read the colour value using COGL_PIXEL_FORMAT_RGBA_8888_PRE

picking: Read the colour value using COGL_PIXEL_FORMAT_RGBA_8888_PRE

In commit c0a553163b I changed the format used to read the picking
pixel to COGL_PIXEL_FORMAT_RGB_888 because it was convenient to avoid
the premult conversion. However this broke picking on GLES on some
platforms because for that glReadPixels is only guaranteed to support
GL_RGBA with GL_UNSIGNED_BYTE. Since the last commit cogl_read_pixels
will always use that format but it will end up with a conversion back
to RGB_888. This patch avoids that conversion and avoids the premult
conversion by reading in RGBA_8888_PRE.

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