Simple implementation of drawPosText

The existing implementation of drawPosText is broken in various subtle
ways, in any case doesn't work with Minikin. This patch just implements
it by drawing a separate run for each Unicode character, which should
have the least surprising results for complex scripts such as Khmer.

Part of b/11750374 Resolve TODO items for Minikin

Change-Id: I874ae3c163f0cbe3cdf0160564fab04305aed5aa