0.8.8c2 - MapView rendering optimization

MapView no longer renders every tile when calling scene()->update().
Only updateTiles() should call scene()->update() directly - all other
require scene updates should use updateTiles(getTopLeftVisibleTile(),
getBottomRightVisibleTile()) to update the scene.

MapTileItems now contain a stack of "render regions", which are
refreshed after every draw. This alleviates the problem of rendering
tens of thousands of tiles when the necessary portions to update are
only a few tiles.

This has introduced a few annoying, yet functionally insignificant bugs:

	*Selections and transformations do not properly render tiles in
	the correct mode.

	*Floating point render errors have been introduced.

	*Alpha-blending is no longer properly updated for portion-draws.

Until these bugs are fixed, the consequences can be cleared after a full
scene draw.