Allow null clear color to disable clearing screen

[Frosty-J]

There may be a better way of going about this, but it's what I'm doing. Tested on desktop and GWT.

Clearing the screen can harm frame rates slightly (relative to how complex your scene is to render) on Intel integrated graphics (especially when not also clearing the depth buffer?), so in cases where the areas not drawn to are static I prefer not to clear it. When using a proper graphics card, it doesn't seem to make much difference.

Click for fun facts for when drawing directly to the screen buffer (not really relevant to this pull request)

• On GWT, you don't need to clear. Not unless config.preserveDrawingBuffer = true, maybe (haven't tested it; don't see much of a need to).
• If only clearing on resize, such as for games that have pillarboxing, it's good to clear in render() a couple times due to double buffering.
• For some games, it may be a good idea to clear anyway (such as in case the player ends up out of bounds and is subjected to it flickering between buffers).