The rendering implementation is needed for using Gio UI with external
window libraries such as GLFW. Expose it in the new package gpu.
Updates #26
Signed-off-by: Elias Naur <mail@eliasnaur.com>
The policy of rendering on a separate goroutine is separate from
the actual rendering. Reflect that by introducing the RenderLoop
type for driving a GPU from a separate goroutine.
Signed-off-by: Elias Naur <mail@eliasnaur.com>