Add ability to foveate a render target

[cabanier]

three.js is moving to a 2 (or more) pass solution.
This creates a problem for foveation since it's only applied to the textures of the swapchain. If we want to preserve foveation, we'd have to first render to the swapchain texture, then copy it to a regular one and then apply the second pass.

I propose that we add a new API to XRWebGLBinding to apply foveation to the current bound target. That way, the first render pass can render to that target and the next one can draw to the swapchain texture.

Proposed new API:
void foveateBoundTexture(GLenum target, float fixed_foveation)

/agenda Add ability to foveate a render target

[cabanier]

cc @AdaRoseCannon @zachernuk @toji to investigate if this approach will work on their XR systems.