Avoid ANGLE y-inversion rendering path

[jdm]

Based on #25425 (comment), we should do the work to avoid the rendering slow path. On reflection, however, it's not clear to me that this will have an impact on immersive mode - we will likely need to implement y-inversion in our webgl backend the same way that webrender performs the transformation, so the real question is whether the presence of the y-inversion in the glBlitFramebuffer calls during webxr's render_animation_frame makes a material difference to the frame throughput or not.

[jdm]

It should be straightforward to figure out if this work is a high priority:

• record FPS for baseline master servo in the paint example
• apply portions of the from patch from jdm@1de0d5d (the webrender y-inversion rendering option and the opengles setup for the uwp port)
• remove the y-inversion in glBlitFramebuffer in https://github.com/servo/webxr/blob/eedc4a5a19f1c8df8c1614ad15009c35a659cab8/webxr/openxr/mod.rs#L576-L613
• record FPS for upside-down servo in the paint example

If there's a meaningful improvement, we should prioritize this.