cogl_read_pixels: Fix the format used in GLES2
cogl_read_pixels: Fix the format used in GLES2

When converting the data in cogl_read_pixels it was using bmp_format
instead of the format passed in to the function. bmp_format is the
same as the passed in format except that it always has the premult bit
set. Therefore the conversion would not handle premultiply correctly.

http://bugzilla.clutter-project.org/show_bug.cgi?id=2414

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