Ensure 2D MSAA resolve is performed when 3D content but no 2D content in scene

[BastiaanOlij]

When 2D MSAA is enabled we will output our 3D render data directly into the MSAA buffer.
If no 2D elements were being rendered, the 2D MSAA buffer was not being resolved if there were no 2D elements being rendered.
This PR keeps track of a flag that gets set when data is written directly into the 2D MSAA buffer, and gets unset when drawing to this buffer commences which will lead to an automatic resolve.
If the flag is still set at the end of rendering a viewport, a resolve is triggered.

Note that if there are no 2D elements while 2D MSAA is enabled, rendering 3D content into the MSAA buffers and resolving causes unnecessary overhead. It is advisable that a developer disabled 2D MSAA when there is no 2D content. A warning is given in this scenario.

Note while not specifically tested, there is reason to believe that this fix will fail on Adreno hardware due to a driver bug. Adreno will skip empty passes, even if that pass triggers a resolve. This behavior was discovered when using subpasses in the mobile renderer. It is not certain this behavior also applies to normal passes. I do not have hardware to test this. The fix is for the developer to disable 2D MSAA when there are no 2D elements to render, which is better practice anyway.

Fixes #84280

[clayjohn]

Tested locally and can confirm that this fixes the reported bug.

The code changes look good.

The actual change is quite low risk, so this should be suitable for merging in rc2.

[akien-mga]

Thanks!