test_mask_to_array: Add a few more test-cases
test_mask_to_array: Add a few more test-cases

videotestsrc-mask-non-black.png has a few white rectangles not touching
each other -- good to have one such case from a PNG.

Also added tests for completely black (empty) masks. These will raise
ValueError: ... doesn't overlap with the frame's Region(...)

We already have a test for a mask with a hole in the middle:
`~Region(2,2,2,2)`.

stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester,stb-tester/stb-tester