OpenXR: Fix crash when using XR_EXT_frame_synthesis after Godot 4.7-dev5

[dsnopek]

Fixes #118902

[BastiaanOlij]

This seems smart regardless, with on_pre_render being called from the rendering thread, it's probably a good idea to use the render state anyway.

[dsnopek]

Looking at this a bit more, I think this could be made simpler by switching from on_pre_render() to on_pre_draw_viewport(). And, we're doing our post-frame stuff in on_post_draw_viewport(), so it would have more symmetry to do the pre-frame stuff in on_pre_draw_viewport() anyway...

[dsnopek]

I've updated this for the on_pre_draw_viewport() / on_post_draw_viewport() symmetry mentioned above

[Repiteo]

Thanks!