Can we add an new interface SDL_RenderTriangleList， so that we can implement BatchRender ourselves.
If you are looking to render 3D you are probably out of luck. According the wiki:
For advanced functionality like particle effects or actual 3D you should use SDL’s OpenGL/Direct3D support or one of the many available 3D engines.
For 2D, SDL does have some batch rendering functions, but not for triangles:
Depending on the data structures in the renderer and the engine I could imagine that something like SDL_RenderCopies and SDL_RenderCopiesEX might be nice to have though.