Bug 1176077 introduced the parameter aDirtyRegion to
DrawPaintedLayerCallback, which allows the callback to recompute the
visibility of all items to be painted in that transaction in a single
go. However, this parameter can not always be determined correctly
when using RotatedBuffer, and using an incorrect value was causing
graphical glitches.
Make the parameter optional, and on null values do not perform the
optimisation. Pass null from ClientPaintedLayer, which uses
RotatedBuffer and was causing problems, but continue to pass the
correct value from other Layer implementations. This optimisation was
most important for tiled layers using progressive paint, so this is
okay.
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