Add runtime import of Window in init_scene

The Window class is imported inside TYPE_CHECKING for type hints, but
needs to be imported at runtime when actually creating a window. This
import is deferred to avoid triggering pyglet display initialization
on headless systems (RTD build).

Author: behackl

manim/renderer/opengl_renderer.py

@@ -539,6 +539,8 @@ def init_scene(self, scene: Scene) -> None:
 
         self.background_color = config["background_color"]
         if self.should_create_window():
+            from .opengl_renderer_window import Window
+
             self.window = Window(self)
             self.context = self.window.ctx
             self.frame_buffer_object = self.context.detect_framebuffer()