Support 3D rotations when drawing text

If a perspective transform is set on the Canvas, drawText() should
not attempt to rasterize glyphs in screen space. This change uses
the old behavior instead (i.e. rasterize the glyphs at the native
font size and apply the transform on the resulting mesh.)

This change also adds an optimization: empty glyphs (spaces) do
not generate vertices anymore. This saves a lot of vertices in text
heavy applications such as Gmail.

Change-Id: Ib531384163f5165b5785501612a7b1474f3ff599