Wraps SDL_SetWindowOpacity (from SDL 2.0.5) which sets the opacity of the window
float indicating the opacity of the window from 0.0 (transparent) to 1.0 (opaque)
dsdl2.SDLException if failed to set the window's opacity
See Implementation
Wraps SDL_SetWindowOpacity (from SDL 2.0.5) which sets the opacity of the window