vrend: optimize sysvalue UBO preparation and upload

Gert Wollny requested to merge gerddie/virglrenderer:rework-sysval-buffer into master
  • The sysvalue UBO is now a member of sub_context, and it is filled at the same time the actual state changes are communicated from the guest, and
  • The sysvalue UBO is only submitted if it actually changed.

This fixes a performance regression that was introduced lately by moving this system state data to a UBO.

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