SubTask - Split shared parts of Window from opengl backend

[bhperry]

Pixel should have a core window struct that manages retrieving information from the configured backend. There is probably a lot to disentangle from the opengl package to get this done (e.g., the Canvas struct, which is currently VERY tied to OpenGL).

This may need to be split out into smaller tasks to be manageable.

• Move input state out of backend #82
• Move joystick state out of backend