Add SDL_VIDEO_DOUBLE_BUFFER support to the Wayland backend.

include/SDL_hints.h

@@ -2213,6 +2213,7 @@ extern "C" {
  * Since it's driver-specific, it's only supported where possible and
  * implemented. Currently supported the following drivers:
  *
+ * - Wayland (wayland)
  * - KMSDRM (kmsdrm)
  * - Raspberry Pi (raspberrypi)
  */