[wip] Add canvas.toDataURL("image/png") compression control

[wip] Add canvas.toDataURL("image/png") compression control

Add an optional compression argument to toDataURL for "image/png"
to control the compression of the output image.

The argument is a number in the range [0.0, ..., 1.0], similar to
the quality argument used for "image/jpeg".  Higher values usually
increase compression at the cost of speed.

TEST=fast/canvas/canvas-toDataURL-png-compression.html
BUG=179289