Different operations to the display tree have different costs. Be
aware of these costs when optimizing your code.
* Changing tiles indices in a TileGrid will update an area
covering them all.
* Changing a palette will refresh every object that references it.
* Moving a TileGrid will update both where it was and where it moved to.
* Adding something to a Group will refresh each individual area it
covers.
* Removing things from a Group will refresh one area that covers all
previous locations. (Not separate areas like add.)
* Setting a new top level Group will refresh the entire display.
Only TileGrid moves are optimized for overlap. All other overlaps
cause sending of duplicate pixels.
This also adds flip_x, flip_y and transpose_xy to TileGrid. They
change the direction of the pixels but not the location.
Fixes#1169. Fixes#1705. Fixes#1923.
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