function has_any_transparency(ctx) {
	// @TODO Optimization: Assume JPEGs and some other file types are opaque.
	// Raster file formats that SUPPORT transparency include GIF, PNG, BMP and TIFF
	// (Yes, even BMPs support transparency!)
	const id = ctx.getImageData(0, 0, main_canvas.width, main_canvas.height);
	for (let i = 0, l = id.data.length; i < l; i += 4) {
		// I've seen firefox give [ 254, 254, 254, 254 ] for get_rgba_from_color("#fff")
		// or other values
		if (id.data[i + 3] < 253) {
			return true;
		}
	}
	return false;
}
@param {CanvasRenderingContext2D} ctx
@returns {boolean} whether the canvas has any translucent pixels (with a stupid margin of error)