vdisp/sdl3: set render clear color

Seems like Wayland driver seem to need to have this set explicitly
for the eventual borders to be cleared correctly with SDL_RenderClear.
Otherwise the eventual margins may not be cleared by SDL_RenderClear
(tested with U25.10 live DVD with llvmpipe software renderer inside VM).

Author: MartinPulec

src/video_display/sdl3.c

@@ -869,6 +869,8 @@ display_sdl3_reconfigure_real(void *state, struct video_desc desc)
                 MSG(ERROR, "Unable to create renderer: %s\n", SDL_GetError());
                 return false;
         }
+        SDL_CHECK(
+            SDL_SetRenderDrawColor(s->renderer, 0, 0, 0, SDL_ALPHA_OPAQUE));
         if (s->vsync) {
                 // try adaptive first, if it doesn't succeed try 1
                 if (!SDL_SetRenderVSync(s->renderer,