Add API to check for region intersection without new allocation

It's quite useful to have an API that allows checking whether two regions intersect very quickly without returning the exact intersection region.

So add a new pixman_region_intersect and pixman_region32_intersect API to pixman that does a simple intersection check returning a boolean value without allocating a new region.

When implementing obscured region culling, this helps speed up culling out elements of the UI significantly.