Fixes#4516
By using the `CAMetalLayer`'s `backgroundColor` property instead of
drawing the background color in our shader, it is always stretched to
cover the full surface, even when live-resizing, and it doesn't require
us to draw a frame for it to be initialized so there's no transparent
flash when a new surface is created (as in a new split/tab).
This commit also allows for hot reload of `background-opacity`,
`window-vsync`, and `window-colorspace`.
ff9414d9ea