[3.x] Make WebXR touch events proportional to render target size, not viewport

[dsnopek]

This is a fix to #56819 which is only in 3.x.

The position of touch events was previously proportional to the viewport. This is problematic when the render target from WebXR is a different size than the viewport - the touches will be in the wrong place.

(Note: This isn't the ideal fix - it would be better if the viewport resized to match the render target, but for some reason that isn't "sticking". I suspect this may require some deeper changes in the HTML5 platform. But that's for a future PR!)

[BastiaanOlij]

Interesting that this should be a problem because there shouldn't be a reason for handheld AR devices to change the render target size away from the viewport. That said, seeing when a viewport is marked as the XR viewport the render target size overrides the viewport size and the viewport size becomes defunct, this solution seems sound.

[akien-mga]

Thanks!

[dsnopek]

That said, seeing when a viewport is marked as the XR viewport the render target size overrides the viewport size and the viewport size becomes defunct

Yeah, the thing is the viewport size isn't "sticking" for some reason, it keeps resetting to the size of the browser window. I think this an HTML5 platform thing, I just haven't had a chance to dig that deep yet. Something for my next PR! :-)