Improve Debug type to allow customization of the debug view

[hecrj]

I think we could extend the Debug type with some methods to allow games to easily customize the debug view.

Here is how it currently looks:

Here are a couple of features that would be nice to have:

• Add additional key/value text. Basically extend the current view. This can be useful for counters (think entity count, loaded chunks count, etc.), player position, additional performance information, etc. Some way to define the position of the text on screen would be nice.
• Add rectangles, once we implement mesh support (Mesh support #4), possibly with customizable color. This will be useful to allow games to debug colliders, hitboxes, quadtrees, etc.

The API should be declarative (i.e. no side-effects until Debug::draw). Feel free to share any more ideas here!