render: Move to a batching system for rendering (work in progress).

include/SDL_hints.h

@@ -1029,6 +1029,31 @@ extern "C" {
  */
 #define SDL_HINT_AUDIO_CATEGORY   "SDL_AUDIO_CATEGORY"
 
+/**
+ *  \brief  A variable controlling whether the 2D render API is compatible or efficient.
+ *
+ *  This variable can be set to the following values:
+ *
+ *    "0"     - Don't use batching to make rendering more efficient.
+ *    "1"     - Use batching, but might cause problems if app makes its own direct OpenGL calls.
+ *
+ *  Up to SDL 2.0.9, the render API would draw immediately when requested. Now
+ *  it batches up draw requests and sends them all to the GPU only when forced
+ *  to (during SDL_RenderPresent, when changing render targets, by updating a
+ *  texture that the batch needs, etc). This is significantly more efficient,
+ *  but it can cause problems for apps that expect to render on top of the
+ *  render API's output. As such, SDL will disable batching if a specific
+ *  render backend is requested (since this might indicate that the app is
+ *  planning to use the underlying graphics API directly). This hint can
+ *  be used to explicitly request batching in this instance. It is a contract
+ *  that you will either never use the underlying graphics API directly, or
+ *  if you do, you will call SDL_RenderFlush() before you do so any current
+ *  batch goes to the GPU before your work begins. Not following this contract
+ *  will result in undefined behavior.
+ */
+#define SDL_HINT_RENDER_BATCHING  "SDL_RENDER_BATCHING"
+
+
 /**
  *  \brief  An enumeration of hint priorities
  */