onscreen: Support multisample based onscreen rendering

This adds support for multisample based rendering of onscreen windows
whereby multiple point samples per pixel can be requested and if the
hardware supports that it results in reduced aliasing (especially
considering the jagged edges of polygons)

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

cogl/cogl-framebuffer-private.h

@@ -51,6 +51,7 @@ typedef struct
 {
   CoglSwapChain *swap_chain;
   gboolean need_stencil;
+  int samples_per_pixel;
 } CoglFramebufferConfig;
 
 struct _CoglFramebuffer