render: Enable clipping for zero-sized rectangles

Battle for Wesnoth apparently relies on being able to disable rendering
of UI elements by setting the clip rectangle to be empty.

Resolves: https://github.com/libsdl-org/SDL/issues/6896
Fixes: 00f05dcf "render: only enable clipping when the rectangle is valid"
Signed-off-by: Simon McVittie <smcv@collabora.com>

src/render/SDL_render.c

@@ -2534,7 +2534,7 @@ int SDL_SetRenderClipRect(SDL_Renderer *renderer, const SDL_Rect *rect)
     int retval;
     CHECK_RENDERER_MAGIC(renderer, -1)
 
-    if (rect && rect->w > 0 && rect->h > 0) {
+    if (rect && rect->w >= 0 && rect->h >= 0) {
         renderer->view->clipping_enabled = SDL_TRUE;
         renderer->view->clip_rect.x = rect->x;
         renderer->view->clip_rect.y = rect->y;