cogl: Use pixel_format_to_gl_with_target in find_best_gl_get_data_format

Fixes cogl_texture_get_data() resorting to the wrong conversions when
extracting the texture data. This notably resulted in RGB/RGBA buffers
copied as-is into BGRA buffers, for instance for the fullscreen animation,
or single-window screenshots of such buffers.

https://bugzilla.gnome.org/show_bug.cgi?id=779234