Skip 2D rendering if stereo enabled and fix couple of MSAA issues

[BastiaanOlij]

Small fix in the viewport logic, when we enable stereoscopic rendering we can't render 2D elements as these lack depth information.

The assumption up till now was that users simply wouldn't add 2D elements to the scene but this has lead to a couple of issues:

• 2D elements could be part of a mixed environment where a UI was presented to the user on screen but not in HMD
• even without 2D elements part of the 2D pipeline still ran and could result in empty render passes.

This fix ensures that 2D is disabled when stereoscopic rendering is applied to a viewport.

Note, it isn't enough to check use_xr, for phone AR (ARKit/ARCore) 2D elements can be used.

edit In testing this fix I ran into issues with MSAA2D which had to be dealt with, that threw me down a rabbit hole that ended up fixing a number of MSAA related issues.

second edit turned out when introducing FSR2 we lost some the logic that turns scaling off when scaling is set to 1.0 and introduce overhead both when FSR1 was set or when running on the mobile renderer.
Added a change that re-instates the old logic, but leaves the option to set scaling to 1.0 with FSR2.

Dumb little test project to test the MSAA settings, run with both Mobile and Forward+ to test:
Test MSAA.zip

[BastiaanOlij]

Note didn't want to classify this as a bug but might be worth putting in 4.2 or at least 4.2.1 (and 4.1.3 if we decide to build that).

With the fixes this now introduces, I suggest we merge this as a bugfix.

[BastiaanOlij]

Requested by @DarioSamo to change this. Not sure if this should be > or >=

[fire]

Does this preserve the 2d controls on a 3d plane usecase for vr?

[BastiaanOlij]

Does this preserve the 2d controls on a 3d plane usecase for vr?

No, this purely stops 2D rendering on the viewport that has use_xr set to true and which have stereoscopic rendering, which never worked consistently to begin with due to 2D content not being aware of multiview.

Rendering 2D content to other viewports which are then displayed on 3D quadmeshes etc. works as before.

[clayjohn]

Looks mostly fine to me. There were a few changes that I am unsure about as the reason for the change is not obvious. So I left a few comments asking for clarification

[clayjohn]

Looks fine to me. I am a little wary about the extra copy when using MSAA, but since FSR can only be used on desktop anyway, it isn't a huge deal for now. At some point we will want to return and see if we can't eliminate that copy though.

[BastiaanOlij]

Looks fine to me. I am a little wary about the extra copy when using MSAA, but since FSR can only be used on desktop anyway, it isn't a huge deal for now. At some point we will want to return and see if we can't eliminate that copy though.

Agreed, it's a bit of an unlikely scenario but I'm sure there is a way to have FSR output into an MSAA buffer.

[akien-mga]

Thanks!

[YuriSizov]

Looks like this change led to some regressions and potentially uncovered other issues. I'd avoid cherry-picking it based on this, especially given that it's not clear how critical the fixes are for 4.1 and if we may find more issues in future. I'll remove the label, but if the rendering team still wants this, keep in mind that we also need to pick #84267.