getImageData function on Context2d

[sjrct]

getImageData is a standard function that should exist on the Context2D object, which is as of now not implemented in browser or on native. We do, after all, use it in at least one occasion.

There is some degree of complication with implementing this in the standard fashion as Uint8ClampedArray does not seem to be defined on native android, at the very least. Although that could be emulated with an array of plain JavaScript numbers.