Render: Allow empty cliprect.

This fixes an issue where an empty cliprect is treated the same as a NULL
cliprect, causing the render backends to disable clipping.

Also adds a new API, SDL_RenderIsClipEnabled(render) that allows you to
differentiate between:
 - SDL_RenderSetClipRect(render, NULL)
 - SDL_Rect r = {0,0,0,0}; SDL_RenderSetClipRect(render, &r);

Fixes https://bugzilla.libsdl.org/show_bug.cgi?id=2504

include/SDL_render.h

@@ -551,6 +551,16 @@ extern DECLSPEC int SDLCALL SDL_RenderSetClipRect(SDL_Renderer * renderer,
 extern DECLSPEC void SDLCALL SDL_RenderGetClipRect(SDL_Renderer * renderer,
                                                    SDL_Rect * rect);
 
+/**
+ *  \brief Get wether clipping is enabled on the given renderer
+ *
+ *  \param renderer The renderer from which clip state should be queried.
+ *
+ *  \sa SDL_RenderGetClipRect()
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_RenderIsClipEnabled(SDL_Renderer * renderer);
+
+
 /**
  *  \brief Set the drawing scale for rendering on the current target.
  *