added "gpu" to the list of possible values of SDL_HINT_RENDER_DRIVER in SDL_hints.h

include/SDL3/SDL_hints.h

@@ -2862,6 +2862,7 @@ extern "C" {
  * - "opengles"
  * - "metal"
  * - "vulkan"
+ * - "gpu"
  * - "software"
  *
  * The default varies by platform, but it's the first one in the list that is