Switch WebXRInterface from using external textures to using the render targets from Godot

[dsnopek]

This has two positive effects:

1. 2D controls on a CanvasLayer will actually render, which is useful for smartphone AR
2. There's less code - deleting lines is always good :-)

This PR is only for 3.x - I tried to make a 4.x version, but it really doesn't make sense. The primary change here is that we are taking an OpenGL texture ID for a texture that Godot created and passing it into the WebXR JS code (rather than the texture being created on the JS end). But in 4.x, there is no OpenGL texture ID to pass in, since we're still waiting for 3D and XR support to be added to the OpenGL renderer in 4.x. I asked @BastiaanOlij on RocketChat and he thought it would be OK to make this 3.x only.

[BastiaanOlij]

Looks good @dsnopek , indeed much simpler this way. The external texture feature only makes sense if it results in Godot rendering into the buffer the XR Compositor will use to render things on screen. As this currently not the case but you're blitting the end result to WebXRs provided framebuffer this is simpler and better.

[akien-mga]

Thanks!