Always enable window_res (#6767)

Author: BMagnu

code/graphics/2d.cpp

@@ -1844,14 +1844,16 @@ bool gr_init(std::unique_ptr<os::GraphicsOperations>&& graphicsOps, int d_mode,
 		}
 	}
 
-	if (!gr_is_viewport_window() && !Cmdline_window_res.has_value()) {
+	if (!Cmdline_window_res.has_value()) {
 		// For whatever reason, it seems that a combination of Win 11 and presumably NVidia GPU's causes weird artifacts.
 		// These artifacts do not appear in windowed mode, or with an attached renderdoc / nvidia nsight.
 		// Similarly using the -window_res command line parameter prevents this.
 		// To the best of our knowledge, this is because all of the aforementioned methods route the rendering through another buffer
 		// (be that a window, a render overlay from nsight, or an FSO-internal buffer) instead of directly rendering to the OS-provided direct screen backbuffer.
 		// As the cost of -window_res is one single blit of a fullscreen buffer, it's probably an acceptable compromise to get rid of render artifacts.
 		// As such, forcibly enable -window_res at the screen resolution here, if we're in fullscreen.
+
+		// Additionally, SDL3+ doesn't work when reading from the GL_FRONT buffers, so we need our own intermediate buffers.
 		Cmdline_window_res.emplace(static_cast<uint16_t>(width), static_cast<uint16_t>(height));
 	}