Add GM to verify that drawX == (path.addX, drawPath)

Add GM to verify that drawX == (path.addX, drawPath)

This demonstrates a new kind of hybrid unit test/GM.
It creates a grid of cells. In each cell, we do two
renders that are expected to produce the same result.
For each cell, we render the two results overlaid,
and highlight any differing pixels in red. Assuming
there is a diff, the area around the largest diff
is drawn zoomed in from both images.

Matching cells are outlined in green, failing cells
are outlined in red. Triaging this GM just involves
answering the question: "Are there any red boxes?"

"Good" example: https://screenshot.googleplex.com/909P3tvS55f.png
"Bad" example: https://screenshot.googleplex.com/oXBWbEKw5ur.png

To get more tests to pass, (and fix an assert
in Ganesh), I've gone ahead and enforced that user
supplied rects (in drawRect and drawOval) are
always sorted once they hit the canvas virtuals.
Currently, drawArc rejects empty ovals, but I added
the same assert to onDrawArc, if we decide to change
the strategy there.

Bug: skia:
Change-Id: I021a18c85e234298e1d29f333662683d996dd42c
Reviewed-on: https://skia-review.googlesource.com/16983
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>