GL_NVX_multiview_per_view_attributes questions/issues

Two things I noticed today with GL_NVX_multiview_per_view_attributes.
1. In the vertex shader stage, shouldn't the declarations for
```
gl_PositionPerViewNV[];
gl_ViewportMaskPerViewNV[];
```
Be inside of the gl_PerVertex block declaration, instead of as standalone variables? In GLSLang they are declared as standalone variables. This both follows the extension, and makes the interface matching more correct. Other stages have them inside the gl_PerVertex block correctly.

2. Isn't the fact the interfaces aren't matching between stages breaking the Interface matching rules in 7.4.1 of the OpenGL spec anyways? This extension has a different input interface for the GS vs the VS output, for example.

Attn: @jeffbolznv @chaocNV

Start by locating GLSLang's GL_NVX_multiview_per_view_attributes declarations and compare the vertex-stage variables with the extension text and OpenGL section 7.4.1. Done means resolving whether the declarations and cross-stage interfaces conform to the specified rules, with focused validation for the affected shader stages.

Written by the indexing model from the issue text.