[main] Use cogl_read_pixels not glReadPixels in clutter-main.c

[main] Use cogl_read_pixels not glReadPixels in clutter-main.c

The debugging function read_pixels_to_file() and _clutter_do_pick were both
directly calling glReadPixels, but we don't wan't Clutter making direct
OpenGL calls and Cogl provides a suitable alternative.  It also means
read_pixels_to_file() doesn't need to manually flip the data read due to
differences in Clutter/Cogl coordinate systems.