Proper fades before round start / after death

[zackthehuman]

There should be fading sequences when the round starts (which there is now, but its implementation is less than good) and after player death (and also after the level is complete). These all need to be implemented.

[zackthehuman]

Wait for #152 to be completed.

[zackthehuman]

This was working but sometimes causes crashing due to destruction of unique_ptr of a substate. Figure out a different way to do this.

[zackthehuman]

This is buggy overall. I propose we do an event-based or callback-based design where fades can be globally requested, and when fades are done the caller can be notified. This removes the need for having weird temporary state for fading in and out, and will result in an overall cleaner design. Fading can be done using a shader so that the palette can be adjusted rather than just drawing a black quad over the screen like we do now.