Fix Jank

[christolliday]

Known issues:
-On Windows, the whole window flickers while resizing.
-On X11 (and probably other platforms) when growing the window the new window area stays black for a while before being drawn.
-Rendering lags when the layout is updated, rendering and layout are not well synchronized.

The core event/rendering loop needs an overhaul to fix these properly.

I'm planning to take these on after trying to fix the other major issues on OSX, so that these issues can hopefully be fixed and tested across all the major platforms.

[Diggsey]

This is likely blocked on these issues in winit:

rust-windowing/winit#276
rust-windowing/winit#266
rust-windowing/winit#231