framebuffer: make _swap_region coords top-left relative

Cogl aims to consistently put the origin of 2D objects at the top-left
instead of the bottom left as OpenGL does, but there was an oversight
and the experimental cogl_framebuffer_swap_region API was accepting
coordinates relative to the bottom left. Cogl will now flip the user's
given rectangles to be relative to the bottom of the framebufffer before
sending them to APIs like glXCopySubBuffer and glBlitFramebuffer.

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

cogl/cogl-framebuffer.h

@@ -333,7 +333,7 @@ cogl_framebuffer_swap_buffers (CoglFramebuffer *framebuffer);
 #define cogl_framebuffer_swap_region cogl_framebuffer_swap_region_EXP
 void
 cogl_framebuffer_swap_region (CoglFramebuffer *framebuffer,
-                              int *rectangles,
+                              const int *rectangles,
                               int n_rectangles);