ae.ui.shell.sdl2: Don't use SDL_PushEvent

SDL_PushEvent seems to no longer be thread-safe in SDL 2.
ae.ui.video.threadedvideo: Move "started" phase after initialization
ae.ui.shell.sdl2: Handle SDL_WINDOWEVENT_MOVED
ae.ui.video.sdl2common: Don't recreate window on every reinitialization
ae.ui.video.sdl2: Add SDL2 renderer (using SDL2 2D rendering API)