libazure: Enable subpixel text in Skia.

Drastically improves text rendering in Servo, especially on non-Retina
displays.
Merge pull request #103 from pcwalton/subpixel

libazure: Enable subpixel text in Skia.
Add methods to allow clipping to rectangles.

This is much faster than creating a path.
Merge pull request #104 from pcwalton/clip-rect

Add methods to allow clipping to rectangles.