Added SDL_SetWindowOpacity() and SDL_GetWindowOpacity().

This is currently implemented for X11, Cocoa, Windows, and DirectFB.

This patch is based on work in Unreal Engine 4's fork of SDL,
compliments of Epic Games.

src/dynapi/SDL_dynapi_overrides.h

@@ -599,3 +599,5 @@
 #define SDL_JoystickFromInstanceID SDL_JoystickFromInstanceID_REAL
 #define SDL_GetDisplayUsableBounds SDL_GetDisplayUsableBounds_REAL
 #define SDL_GetWindowBordersSize SDL_GetWindowBordersSize_REAL
+#define SDL_SetWindowOpacity SDL_SetWindowOpacity_REAL
+#define SDL_GetWindowOpacity SDL_GetWindowOpacity_REAL

src/dynapi/SDL_dynapi_procs.h

@@ -633,3 +633,5 @@ SDL_DYNAPI_PROC(SDL_GameController*,SDL_GameControllerFromInstanceID,(SDL_Joysti
 SDL_DYNAPI_PROC(SDL_Joystick*,SDL_JoystickFromInstanceID,(SDL_JoystickID a),(a),return)
 SDL_DYNAPI_PROC(int,SDL_GetDisplayUsableBounds,(int a, SDL_Rect *b),(a,b),return)
 SDL_DYNAPI_PROC(int,SDL_GetWindowBordersSize,(SDL_Window *a, int *b, int *c, int *d, int *e),(a,b,c,d,e),return)
+SDL_DYNAPI_PROC(int,SDL_SetWindowOpacity,(SDL_Window *a, float b),(a,b),return)
+SDL_DYNAPI_PROC(int,SDL_GetWindowOpacity,(SDL_Window *a, float *b),(a,b),return)