compositor: reduce the number of triangles

compositor: reduce the number of triangles

The intersection of two rectangles is guaranteed to be convex. Therefore
we do not need a center vertex for the triangle fan, we can simply use
the whatever first vertex the intersection polygon has. This reduces the
number of triangles, while still painting the exact same area.

While at it, emit_vertex() nested function is factored into the
for-loop, since that is the only calling site left.

Comments are updated to reflect the changes, and some unrelated comment
fixes are in repaint_region().

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Rob Clark <rob.clark@linaro.org>