Start moving to the new canvas structure.

Adds DrawingMethods with some of the methods it defines. Context is now
an implementation of DrawingMethods.

The sample.js file now shows how the context is used.

Not much new code here, that's mostly in DrawingMethods::DrawPath, most everything else is a code move.

BUG=skia:

Review URL: https://codereview.chromium.org/676423002
Cut down SkBBH API more.
  - The expected case is now a single bulk-load insert() call instead of N;
  - reserve() and flushDeferredInserts() can fold into insert() now;
  - SkBBH subclasses may take ownership of the bounds

This appears to be a performance no-op on both my Mac and N5.  I guess
even the simplest indirect branch predictor ("same as last time") can predict
the repeated virtual calls to SkBBH::insert() perfectly.

BUG=skia:

Review URL: https://codereview.chromium.org/670213002
Fix the way we patch up the matrix for scaled images that aren't
clamp/clamp

BUG=skia:2904
TBR=reed

Review URL: https://codereview.chromium.org/675823002
add gm: filterbitmap_image_color_wheel

BUG=skia:3061

TBR=mtklein@google.com

Review URL: https://codereview.chromium.org/633333004