Frequently, excludeClipRectangle will be called several times in a row,
in order to trim away borders on each side of a rectangle. When this
happens, we want to avoid creating geometric clip layers which exclude
only two or three of the borders, and instead wait until all borders
have been excluded before applying the clip list. This way, it may be
possible to simplify the clip list to a single rectangle, which can be
implemented using the faster axis-aligned clipping layer.
This assertion is intended to mirror the behaviour of an
informational/performance diagnostic message raised by the D2D debugging
layer.
It seems the D2D diagnostic is raised when the proposed clip region is
aligned to the screen, not to the current transform.
Before this change, the assertion could incorrectly fire when clipping
to transformed rectangles. This could be seen when clicking the
star-shaped buttons in the ComponentTransformsDemo.
With this change in place, the assertion will still fire when e.g.
calling Graphics::reduceClipRegion on a screen-aligned rectangular path,
but will not fire when this path is skewed/rotated etc.
