A tile is about to be updated (it is either about to be grabbed
for writing, or it is being released from writing).
Parameters:
-
source - the image that owns the tile.
-
tileX - the X index of the tile that is being updated.
-
tileY - the Y index of the tile that is being updated.
-
willBeWritable - If true, the tile will be grabbed for writing;
otherwise it is being released.