Added classes SkPatch and SkPatchMesh which help encapsulate and generalize this...

Added classes SkPatch and SkPatchMesh which help encapsulate and generalize this new primitive. The functionality and responsability of each class is better explained in the comments of the files.

Each patch defines a method genMesh that produces the geometry to draw. To do this they receive a SkPatchMesh object which they need to initialize in order to set up how the data is going to be formatted. Later they call function like setColor or pointAt to set the values at a specific index, the SkMeshPatch object handles the indices based on the format and makes it transparent to the client.

Added a slide to sample app to show how to set up this classes and how they interact.

BUG=skia:
R=jvanverth@google.com, egdaniel@google.com, bsalomon@google.com

Author: dandov@google.com

Review URL: https://codereview.chromium.org/405163003