Refactor Skia shaders handling.

With this change, Skia shaders can easily be applied to any mesh. This change also
supports ComposeShader. For instance, this can be used to blend a gradient and a
bitmap togehter and paint a string of text with the result.

Change-Id: I701c2f9cf7f89b2ff58005e8a1d0d80ccf4a4aea