Get the opacity of a window.
int SDL_GetWindowOpacity(SDL_Window * window, float * out_opacity);
|window||the window to get the current opacity value from|
|out_opacity||the float filled in (0.0f - transparent, 1.0f - opaque)|
Returns 0 on success or a negative error code on failure; call SDL_GetError() for more information.
If transparency isn't supported on this platform, opacity will be reported as 1.0f without error.
opacity is ignored if it is NULL.
This function also returns -1 if an invalid window was provided.
This function is available since SDL 2.0.5.