test-cogl-readpixels: Reset the viewport and matrices

The on_paint function for test-cogl-readpixels tries to temporarily
set the projection, modelview and viewport to its own values. However
it was never restoring the saved values so it could affect the results
of subsequent tests.

tests/conform/test-cogl-readpixels.c

@@ -124,6 +124,13 @@ on_paint (ClutterActor *actor, void *state)
 
   cogl_handle_unref (tex);
 
+  /* Restore the viewport and matrices state */
+  cogl_set_viewport (saved_viewport[0],
+                     saved_viewport[1],
+                     saved_viewport[2],
+                     saved_viewport[3]);
+  cogl_set_projection_matrix (&saved_projection);
+  cogl_pop_matrix ();
 
   /* Comment this out if you want visual feedback of what this test
    * paints.