patches and uses 4 private arrays to store the values of the control points and
colors. When it needs a patch at a certain position of the grid it just
builds it using the corresponding values of the array and the
grid coordinates provided. Details on implementation are documented in the corresponding classes' comments.
Also added a gm for mesh gradients.
BUG=skia:
R=egdaniel@google.com, reed@google.com
Author: dandov@google.com
Review URL: https://codereview.chromium.org/451723003
