framebuffer: split out GL read_pixels code

This moves the direct use of GL in cogl-framebuffer.c for handling
cogl_framebuffer_read_pixels_into_bitmap() into
driver/gl/cogl-framebuffer-gl.c and adds a
->framebuffer_read_pixels_into_bitmap vfunc to CoglDriverVtable.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

(cherry picked from commit 2f893054d6754e6bc7983f061b27c7858f1a593c)

cogl/driver/nop/cogl-framebuffer-nop.c

@@ -108,3 +108,14 @@ _cogl_framebuffer_nop_draw_indexed_attributes (CoglFramebuffer *framebuffer,
                                                CoglDrawFlags flags)
 {
 }
+
+CoglBool
+_cogl_framebuffer_nop_read_pixels_into_bitmap (CoglFramebuffer *framebuffer,
+                                               int x,
+                                               int y,
+                                               CoglReadPixelsFlags source,
+                                               CoglBitmap *bitmap,
+                                               CoglError **error)
+{
+  return TRUE;
+}