Non-normatively mention shader caches as user agent state

Any shader or program caches that the user agent manages underneath the WebGL implementation need to be considered user agent state that must be partitioned between sites. The rate at which shaders are compiled or programs are linked can provide side-channel information about what sites a user has visited.

References https://github.com/privacycg/storage-partitioning/pull/23 .

Start by reviewing the issue text and the referenced privacycg/storage-partitioning pull request 23 for the relevant user-agent-state terminology. Update the WebGL specification's prose to non-normatively mention shader and program caches as state partitioned between sites, while preserving the side-channel rationale.

Written by the indexing model from the issue text.