Fix Text appears on top of existing text when composing in gmail.

Fix for bug 6473708. This patch changes from "last update wins" to
merging together the change regions, in the logic of deciding which
blocks need to be updated for painting, so that when there are multiple
changes batched for a draw, they're all taken into account.

Change-Id: I183d453c436125e5efec7031b4d61b43989653f9