Update types for `drawCanvas2D`

This is the current function signature in TypeScript:

drawCanvas2D(pos: Vector2, size: Vector2, angle: number, mirror: boolean, drawFunction: Function, screenSpace?: boolean, context?: CanvasRenderingContext2D): void

I'd like to propose changing it to

type CanvasContext = CanvasRenderingContext2D | OffscreenCanvasRenderingContext2D;
function drawCanvas3D<C extends CanvasContext | undefined>(
  pos: Vector2,
  size: Vector2,
  angle: number,
  mirror: boolean,
  drawFunction: C extends undefined
    ? (ctx: CanvasRenderingContext2D) => void
    : (ctx: C) => void,
  screenSpace?: boolean,
  context?: C
): void

This allows for the passing of an offscreen canvas, as well as a regular canvas. It also improves the typing of drawFunction such that ctx will now be correctly typed and inferred based on what is passed to context. If nothing is passed, if defaults to CanvasRenderingContext2D since LJS will use the mainCanvasContext by default.

I'd be happy to make a PR if this is deemed a worthy improvement.

KilledByAPixel / LittleJS

Update types for `drawCanvas2D` #108