Request 24bit depth buffer

Thanks Alug for the fix

src/sdl/i_video.cpp

@@ -1573,6 +1573,11 @@ static SDL_bool Impl_CreateWindow(SDL_bool fullscreen)
 
 #ifdef HWRENDER
 	flags |= SDL_WINDOW_OPENGL;
+
+	// Without a 24-bit depth buffer many visuals are ruined by z-fighting.
+	// Some GPU drivers may give us a 16-bit depth buffer since the
+	// default value for SDL_GL_DEPTH_SIZE is 16.
+	SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
 #endif
 
 	// Create a window