Add debug callbacks

[GammaGames]

Fixes #53

Is playdate.debugDraw called on hardware? I don't have a devices so I can't test

[Mark-LaCroix]

This is good. I'll have to run some tests before pulling, just to be sure there's no overhead, or it's worth it even if there is. It might be a bit before I do another sprint, though. Happy to hear from others if they want to take a crack at seeing how this runs/works on device.

[GammaGames]

If there is overhead you could always check pd.isSimulator before enabling the callback

[ericlewis]

I integrated this into my local tree (it's a great feature — fixes #53) and hit a couple of issues worth flagging:

1. The debugColor default crashes on scene enter. Graphics.kColorWhite is a number constant, but enter() calls playdate.setDebugDrawColor(table.unpack(self.debugColor)), so any scene that doesn't override debugColor with a table crashes as soon as it enters. A {r, g, b, a} table default (e.g. {1, 0, 0, 1}) matches what setDebugDrawColor() expects.

2. disableDebug() resets Noble.showFPS. Setting Noble.showFPS = false on every scene exit silently turns off the FPS display for games that enabled it, which feels like an unrelated side effect — probably better to leave showFPS alone.

3. Re: the overhead question above — gating on playdate.isSimulator (as @GammaGames suggested) works nicely, since playdate.debugDraw() isn't rendered on hardware anyway. Setting playdate.debugDraw = nil in disableDebug() (instead of an empty function) also lets the runtime skip the call entirely.

FWIW, here's the shape I ended up with locally:

NobleScene.debugColor = {1, 0, 0, 1}

function NobleScene:enableDebug()
	if (playdate.isSimulator) then
		playdate.setDebugDrawColor(table.unpack(self.debugColor))
		playdate.debugDraw = function()
			self:drawDebug()
		end
	end
end

function NobleScene:disableDebug()
	playdate.debugDraw = nil
end

with enter() calling self:enableDebug() and exit() calling self:disableDebug() as in this PR. Happy to share the full diff if useful.