Revert "Revert "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: I552e2c2e4713a5ea68801fdb8e3bab3867e77988
Reviewed-on: https://skia-review.googlesource.com/44261
Reviewed-by: Robert Phillips <robertphillips@google.com>
5 files changed