Add multitexture support to text and path shaders

This does not add additional textures to the atlases, just adds the
ability to access those textures in the shaders.

Bug: skia:3550
Change-Id: I636b329a6f748b6753f5f92a70066fb412623df2
Reviewed-on: https://skia-review.googlesource.com/43000
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
5 files changed