Properly make use of frame index

[MeineZ]

Properly make use of the frame index in the renderer to avoid lagging behind 3 frames. As of right now, it seems like the Wisp viewport is one frame behind on Maya.

For example, it starts with a black screen even though it already rendered a frame. However, this frame is still in the back buffer and not presented to the screen. As soon as a new update is detected this frame is shown and the next frame is rendered, though, it's not presented yet.

Making use of the frame index properly fixes this issue.

Very noticeable when zooming in and out very fast.

[tntmeijs]

Frame indexing has been implemented by #95. Will look into this and texture deallocation next.

[sainsay]

Lag has been reduced significantly in #129