The FontDrawer used in TextEdit was previously not being flushed before drawing auto-completion boxes. This was causing rendering artifacts.
This PR also increases the backward compatibility of the MultiRect OFF mode, by forcing a flush after each character.
Large groups of similar rects can be processed more efficiently using the MultiRect command. Processing common to the group can be done as a one off, instead of per rect.
Adds the new API to VisualServerCanvas, and uses the new functionality from Font, BitmapFont, DynamicFont and TileMap, via the VisualServerCanvasHelper class.