VideoCommon: Fix stereoscopic 3D on OpenGL < 4.3 (macOS)

[TellowKrinkle]

gl_Layer isn't accessible from fragment shaders until OpenGL 4.3, so pass it separately

Is this worth adding a feature flag or should we just do it for all OpenGL?

[iwubcode]

While this is rather minor and I have very little say in the direction of Dolphin, I'd still suggest not to do this. OGL has been a bit of annoyance to me, so please consider this a small rant, not necessarily directed at you @TellowKrinkle , just a general frustration.

OpenGL 4.3 was released in 2012. That means this change is targeting machines that lack support over 10 years old. Is it worth adding additional complexity (as small as it may be) for such old hardware? How will we ever shed technical debt if we continue adding changes like this?

I personally want us to update the default OpenGL version, as it's cumbersome when we need something to throw a bunch of extensions and check for them. I think our code assumes version 3.x or possibly earlier.

If we're going to continue with this PR, we should add a flag based on principle because that's the approach we've opted to take.

[Wowfunhappy]

As things stand right now, Dolphin on macOS is horribly broken when stereoscopic 3D is enabled and OpenGL is the selected backend. The user has no way of knowing which settings caused the breakage.

If this isn't going to be fixed, then Dolphin's UI should be updated to gray out the stereoscopic 3D options when OpenGL is the selected backend on macOS, and vice versa. Or if that isn't feasible, Dolphin should at least show a less confusing error message when a game is run with these incompatible settings selected.

Personally, I'd prefer that the problem be fixed so that macOS users can use stereoscopic 3D under OpenGL. However, I'm not a Dolphin developer, so I recognize I'm not exposed to these sorts of code maintenance concerns.

[AdmiralCurtiss]

I'd be fine with merging this but it's currently got a conflict.

[OatmealDome]

Agreed with @Wowfunhappy. We can get rid of this change when OpenGL support on macOS is dropped (wasn't there some discussion about that on the Discord?).