The number of updated tiles was not reset at the end of a progressive block
treatment leading to possibly overflow the updatedTiles array. This patch also
introduces a dirty bit on tiles, so that a tile updated multiple times is just
mark once as modified.
This is a fix for #8816. When PROGRESSIVE_SURFACE_CONTEXT.tiles were reallocated, we were
ending up with wrong tiles in PROGRESSIVE_BLOCK_REGION.tiles when the memory block was moved.
