metal: Deal better with -[CAMetalLayer nextDrawable] returning nil.

Fixes #4863.

include/SDL_render.h

@@ -1806,9 +1806,14 @@ extern DECLSPEC void *SDLCALL SDL_RenderGetMetalLayer(SDL_Renderer * renderer);
  * This function returns `void *`, so SDL doesn't have to include Metal's
  * headers, but it can be safely cast to an `id<MTLRenderCommandEncoder>`.
  *
+ * Note that as of SDL 2.0.18, this will return NULL if Metal refuses to
+ * give SDL a drawable to render to, which might happen if the window is
+ * hidden/minimized/offscreen. This doesn't apply to command encoders for
+ * render targets, just the window's backbacker. Check your return values!
+ *
  * \param renderer The renderer to query
  * \returns an `id<MTLRenderCommandEncoder>` on success, or NULL if the
- *          renderer isn't a Metal renderer.
+ *          renderer isn't a Metal renderer or there was an error.
  *
  * \since This function is available since SDL 2.0.8.
  *