draw text onto an image.
This is a simple API that doesn't handle line breaking, ligatures,
right-to-left or vertical scripts, and other fancy features.
R=r, rsc
CC=golang-dev
http://codereview.appspot.com/1121045
Parsing a malformed TTF may result in an index out-of-bounds panic,
rather than guaranteeing to return an error. This may be tightened
in the future.
R=r, rsc
CC=golang-dev
http://codereview.appspot.com/940044
