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

--HG--
extra : amend_source : 9e5ac76e3f009d9ae49bc61c350df3ba891267b5
extra : histedit_source : b92b8be4d05b19a89fa0dee57f7ed6b1e924cb99%2Cce419f6ade87bafc78ff42702c1f263d2469e7e7

src/render/SDL_sysrender.h

@@ -143,6 +143,10 @@ struct SDL_Renderer
     SDL_Rect clip_rect;
     SDL_Rect clip_rect_backup;
 
+    /* Wether or not the clipping rectangle is used. */
+    SDL_bool clipping_enabled;
+    SDL_bool clipping_enabled_backup;
+
     /* The render output coordinate scale */
     SDL_FPoint scale;
     SDL_FPoint scale_backup;