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Test which verifies null filters and filter regions.

Four subtests each consist of a small red circle overdrawn with a larger black circle. Filters are applied to three of the black circles, hiding them and showing the red circle. The correct rendering is a black circle at top left and red circles in the other three cases.

The topleft subtest has no filter applied to the black circle, so the black circle is visible and the red one is not. The top right subtest applies a filter to the black circle, but there is no corresponding filter element. Thus, a null filter is applied and the black circle is not shown, allowing the red circle to be seen.

The bottom left subtest applies an empty filter element with the default filterRegion, and the bottom right subtest applies an empty filter with a non-default filterRegion. In both cases where empty filters are applied, the result of the filter is a transparent black offscreen, thus showing the red circle underneath.