test-backface-culling: don't call _swap_buffers
test-backface-culling: don't call _swap_buffers

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 <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

gcampax/cogl,djdeath/cogl-android,Distrotech/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl-android,djdeath/cogl,djdeath/cogl,gcampax/cogl,Distrotech/cogl,djdeath/cogl,collects/cogl,collects/cogl,Distrotech/cogl,Distrotech/cogl