Use correct (guessed) VC non-alpha image format

Following your clue from your clutter-list e-mail, I guessed - seemingly correctly - the fix for the blueish colors.

The formats using alpha were correct, my problem was that COGL selected non-alpha formats. Since RGBA32 worked, I guessed for the non-alpha version we were supposed to use RGBX32. I verified it with the cogl-hello test program, worked nicely.