Fix XR rendering in 'opengl3' driver and expose true size via the Viewport node

[dsnopek]

This PR supercedes #65524 which superceded PR #64513

Yesterday, @BastiaanOlij and I had a call, and decided to return to the same basic approach as PR #64513, but adding in the improvements to Viewport nodes when using XR from PR #65524

It does 3 things:

• It fixes the bug where the render buffers would get out-of-sync with the render targets in the 'opengl3' renderer that PR [opengl3] Prevent viewport's render buffers getting out of sync with render target when resized by XR interface #64513 aimed to fix
• It improves how Viewport nodes expose their size when entering and exiting XR mode (similar to what PR Resize Viewport for XR in the Viewport node #65524 aimed to do)
• It adjusts to the changes that @clayjohn made in PR Fix multiwindow support in GLES3 for X11, Windows, and MacOS. #65727 so that XR will render correctly in the 'opengl3' renderer

I've tested this with the 'opengl3' renderer outside of XR, and in XR with both the MobileVRInterface and WebXRInterface (needs PR #64514 on top of this one), as well as with the mobile vulkan renderer in XR with the OpenXRInterface (Oculus Link with a Quest 2). Everything seemed to work fine!

Fixing this bug is necessary to start getting WebXR working again on Godot 4

[BastiaanOlij]

This to me looks great

[BastiaanOlij]

Do note that the OpenGL3 driver doesn't have multiview/stereoscopic support yet, that's probably the next thing we will need to look into before we can make WebXR actually work.

[dsnopek]

Well, WebXR also supports smartphone AR, which isn't stereoscopic! So, it's still worth to fix up WebXR, even without stereo. But, yes, we'll need stereo for VR!

[BastiaanOlij]

I'd like to run through a test or two before we merge this just in case we overlooked something, but won't have time for that today sadly, but I will by the weekend.

[clayjohn]

The changes seem fine to me. If Bastiaan tests and is happy with the result then its good to merge

[BastiaanOlij]

As far as I can tell this is working fine so I think we can merge this :)

[clayjohn]

Great work! Thank you