Add clearCanvas option

[lukix]

This can be useful when wanting to combine view from 2 cameras on one canvas. For example one for rendering game objects and one for rendering UI elements.

draw({
  context,
  objects: gameObjects,
  camera, // dynamic game camera (for example following character)
});

draw({
  context,
  objects: uiObjects,
  clearCanvas: false, // we want to overlay UI on top of previously rendered game objects
  // using default static camera to position UI elements independently from game camera
});