get_brush_canvas_implementation = (brush_shape, brush_size) => {
	const canvas_size = get_brush_canvas_size(brush_size, brush_shape);

	const brush_canvas = make_canvas(canvas_size, canvas_size);

	// brush_canvas.ctx.fillStyle = brush_canvas.ctx.strokeStyle = "black";
	render_brush(brush_canvas.ctx, brush_shape, brush_size);

	return brush_canvas;
}
Gets the canvas for a brush.

USAGE NOTE: must be called outside of any other usage of op_canvas (because of render_brush)
@TODO: protect against browser clearing canvases, invalidate cache

@param {BrushShape} brush_shape
@param {number} brush_size
@returns {PixelCanvas}