Add a test for cogl_framebuffer_push_path_clip

The test makes an L-shaped path that fills the whole framebuffer
except for the top right quadrant. It then clips to that and tries to
fill the framebuffer with a rectangle. Then it verifies that all of
the quadrants have the expected colour.

This is currently failing due to a bug in the primitive clipping.

Reviewed-by: Robert Bragg <robert@linux.intel.com>
Fix calculating the bounds when clipping from a primitive

When projecting the bounding rectangle of a primitive it was using the
modelview matrix twice instead of the modelview and projection
matrices so it was coming out with garbage.

Reviewed-by: Robert Bragg <robert@linux.intel.com>