Currently, we gamma correct colors on every clear and every draw.
It's taking a lot of time. Instead, we'll gamma correct colors
when they're changed using lovr.graphics.setColor/setBackgroundColor.
Having a normal Canvas object that represents the backbuffer reduces
some indirection where we have to last-minute check if a Canvas is
set. It also means that all of the draw-related info that was _sometimes_
on the Canvas is now _always_ on the Canvas, which reduces the amount
of redundant information we need to provide for a draw call.
There may be some issues related to changing the width/height/stereo
of the default Canvas.
- Ref struct only stores refcount now and is more general.
- Proxy stores a hash of its type name instead of an enum.
- Variants store additional information instead of using a vtable.
- Remove the concept of superclasses from the API.
- Clean up some miscellaneous includes.