Wraps SDL_GetWindowOpacity (from SDL 2.0.5) which gets 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 get the window's opacity
See Implementation
Wraps SDL_GetWindowOpacity (from SDL 2.0.5) which gets the opacity of the window