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

GNOME/clutter,ebassi/clutter,jigpu/clutter,ebassi/clutter,heysion/clutter-clone,djdeath/clutter-multithreaded,nobled/clutter,ebassi/clutter,djdeath/clutter,rkudiyarov/clutter_osx,djdeath/clutter,Distrotech/clutter,Distrotech/clutter,djdeath/clutter,spatulasnout/clutter,djdeath/clutter-android,djdeath/clutter-android,nobled/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,heysion/clutter-clone,collects/clutter,Distrotech/clutter,kerrickstaley/clutter-vala,jigpu/clutter,jigpu/clutter,djdeath/clutter-multithreaded,ebassi/clutter,rkudiyarov/clutter_osx,rkudiyarov/clutter_osx,heysion/clutter-clone,djdeath/clutter-android,Distrotech/clutter,GNOME/clutter,dlespiau/clutter,heysion/clutter-clone,jigpu/clutter,collects/clutter,spatulasnout/clutter,collects/clutter,dlespiau/clutter,GNOME/clutter,djdeath/clutter-multithreaded,heysion/clutter-clone,jigpu/clutter,collects/clutter,nobled/clutter,dlespiau/clutter,djdeath/clutter,kerrickstaley/clutter-vala,spatulasnout/clutter,rkudiyarov/clutter_osx,djdeath/clutter-android,GNOME/clutter,spatulasnout/clutter,jigpu/clutter,kerrickstaley/clutter-vala,GNOME/clutter,djdeath/clutter-android,djdeath/clutter,collects/clutter,ebassi/clutter,spatulasnout/clutter,djdeath/clutter-multithreaded,djdeath/clutter,Distrotech/clutter