Made a more general mechanism for getting a canvas callback during GUI
rendering, clipped to a region. Use that to implement the zoom window.
The new version has better zoom control (I think), avoids bugs with
clamped image drawing stretching off-canvas, adds a highlight of the
pixel under the cursor, and also prints the RGBA values of that pixel.
Bug: skia:
Change-Id: I2c4da581648e7923c2a6fb28846dfdb52bdd3029
Reviewed-on: https://skia-review.googlesource.com/70723
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
ead517d822