Imgui integration

Allow programs to set an imgui overlay, for simple UI creation. This will necessarily expose the imgui-rs API, so we should make very explicit that this part of the API is unstable, and we may bump the imgui-rs version to a non-backwards compatible version without changing the pxl major version.

The alternative is to bump the pxl major version whenever imgui-rs releases a non-backwards compatible change, which may be preferable.