Add utility for converting SkCanvas to CGContext
Skia on Mac uses Skia to render WebKit, and CG
to render UI elements. The CG elements need a
transcribed graphics context that preserves the
canvas matrix, and the canvas clip.
The BitLockerSkia utility class sets up a CGContext
from the SkCanvas, locks the bitmap's bits, and
releases the lock when the class goes out of scope.
Each time the CGContext is retrieved, it is rebuilt.
This permits the caller to modify the same canvas
Outside of the unit test, there are no callers
to this utility for now.