cogl: Only do the swizzling hack for GL contexts

The GL_BGRA definition is not available for GLES2 contexts, which use
the EXT_texture_format_BGRA8888 instead, causing a build failure when
trying to use it in those contexts.

Fortunately, this hack is only relevant for GL, so let's guard it to
prevent the failure in GLES2, where that extension is used instead.

https://bugzilla.gnome.org/show_bug.cgi?id=786568