MaskBlt should check use of pattern.

[^MaskBlt-check-use-of-pattern]
This patch make MaskBlt to check use of pattern.

before: MaskBlt: 45 tests executed (0 marked as todo, 1 failure), 0 skipped.
after: MaskBlt: 45 tests executed (0 marked as todo, 0 failures), 0 skipped.