Add support for debug drawing

[MoritzBrueckner]

This PR adds support to use Bullet's debug drawing interface from Haxe.

JS implementation

• To make strings work in JS, I had to update ammo.js to get access to the Ammo.UTF8ToString() function. This should not break anything hopefully.

HL implementation

• Since neither Haxe/HL, webidl nor the newer https://github.com/onehundredfeet/hl-idl seem to support implementing C++ interfaces on the Haxe side, I implemented the btIDebugDraw interface in C++.
• Contrary to JS where we can instantiate multiple DebugDrawer instances, the C++ side uses a singular global debug drawer instance to avoid passing around instance pointers via Haxe (which would be a bit messy). Because the PhysicsWorld trait sets the debug mode when it's instantiated and it is instantiated again when switching scenes, the global instance is updated correctly when switching scenes.