Snapcraft: Clarify the opengl interface reference

[akcano]

Update the following reference page:

The opengl interface

Background

The documentation for the opengl interface doesn't mention that it should be referenced in the Snapcraft YAML file to enable GPU acceleration for CUDA etc.; otherwise, for example, CUDA libraries may appear accessible for import inside the snap, but actual GPU interaction may fail due to permission issues.

Prerequisites

Familiarity with Snapcraft or snaps, and probably GPU pass-through mechanics, is needed to understand the content and test any conjectures. The documentation is Discourse-hosted, so some experience with Discourse is also a plus.