Add support for (GLX|WGL)_EXT_swap_control_tear.

This required a small public API change: SDL_GL_SetSwapInterval() now accepts
 negative values, and SDL_GL_GetSwapInterval() doesn't report errors anymore
 (if it can't work, it'll return zero as a reasonable default).

If you need to test for errors, such as a lack of swap_control_tear support,
 check the results of SDL_GL_SetSwapInterval() when you set your desired
 value.

src/video/directfb/SDL_DirectFB_opengl.c

@@ -250,8 +250,7 @@ DirectFB_GL_SetSwapInterval(_THIS, int interval)
 int
 DirectFB_GL_GetSwapInterval(_THIS)
 {
-    SDL_Unsupported();
-    return -1;
+    return 0;
 }
 
 void