Add renderer motion vectors API for use with OpenXR

[devloglogan]

This is the renderer-agnostic portion of #97151. That PR will remain for the compatibility renderer implementation portion. This PR lays the framework for OpenXR to provide motion vector related render targets to the renderer, allowing for the use of application space warp.

If you're interested in testing it out, you'll have to build using either the compatibility (#97151) or mobile (#100283) renderer PRs that build on top of this one. The XR_FB_space_warp OpenXR extension can then be utilized by building the OpenXR vendor repo with GodotVR/godot_openxr_vendors#222.

[dsnopek]

Thanks!

I already reviewed these changes as part of PR #97151 (before the changes were moved here) and, skimming through this PR, everything still looks good to me :-)

[clayjohn]

Rendering API changes look great to me!

We already discussed them in the original PR, so I think this is safe to go ahead

[Repiteo]

Thanks!