Improve Blender Lighting Hack

[sunjay]

It's currently really hard to match the colors produced by a Blender-exported 3D model. It's possible that this isn't Blender-specific, but that's the main 3D program we're targeting right now.

Notice how different the colors in these two images are:

The left is a render produced by Blender and the right is an image produced by spritec (without the intensity hack in #193). The colors having different brightnesses indicates a lighting issue to me. While the colors are certainly affected by our use of toon shading, I think a larger issue at play is the lighting hack we had to implement because of a bug in Blender's glTF exporter:

spritec/src/scene/light_type.rs

Lines 66 to 69 in 7404bf9

	// HACK: The glTF exporter for Blender has a bug where it exports the light intensity in
	// the wrong units. This works around that issue.
	// See: https://github.com/KhronosGroup/glTF-Blender-IO/issues/564
	let intensity = light.intensity() / 1000.0;

I chose to divide by 1000.0 because Blender lights start at 1000 W and this makes the default light have an intensity of 1.0. In practice however, this is not a good approximation. We should play around with the way we normalize the Blender light values. Since Watt is a unit of power, maybe the relationship is actually exponential? (i.e. maybe we should take the log?)

Another possible solution: Maybe we should just give in and implement glTF's physically-based lighting model?