support offscreen drawing

Redirecting drawing operations to another SDL surface is useful for postprocessing effects, procedurally generated sprites, etc.

Proposed API:

canvas = createCanvas(width, height)
canvas = loadImage(filename)
previous = selectCanvas(canvas)
drawCanvas(canvas, x, y)
canvas = rotozoomCanvas(srcCanvas, angle, zoom, smooth)

The implementation would mostly be a renaming of the existing sprite code and adding more bindings. The sprite() function could be reimplemented in Lua in terms of the canvas functions.

Let me know what you think of this API. I'll implement it in the next couple of days.