Fix -Werror=maybe-uninitialized in GLES2 on release builds

Issue introduced in #28796.

drivers/gles2/rasterizer_scene_gles2.cpp

@@ -2695,7 +2695,8 @@ void RasterizerSceneGLES2::render_scene(const Transform &p_cam_transform, const
 	Environment *env = NULL;
 
 	int viewport_width, viewport_height;
-	int viewport_x, viewport_y;
+	int viewport_x = 0;
+	int viewport_y = 0;
 	bool probe_interior = false;
 	bool reverse_cull = false;