This removes the call to cogl_framebuffer_swap_buffers in the test's
paint() function since we shouldn't assume that the framebuffer is a
CoglOnscreen framebuffer - in fact by default the framebuffer is
offscreen. The swap was resulting in a crash since
cogl_framebuffer_swap_buffers has started asserting that the given
framebuffer is a CoglOnscreen.
Reviewed-by: Neil Roberts <neil@linux.intel.com>