Adds ability to draw rects using a unit square vertex buffer. Useful when matrix...

Adds ability to draw rects using a unit square vertex buffer. Useful when matrix/uniform changes are less expensive than sending new verts.

Adds optional matrix parameters to GrContext drawRect and drawRectToRect so that non-axis-aligned matrices can be drawn using these functions.

codereview Issue 4105049

git-svn-id: http://skia.googlecode.com/svn/trunk@749 2bbb7eff-a529-9590-31e7-b0007b416f81