clean up vtest resource support

Merged Chia-I Wu requested to merge olv/virglrenderer:vtest-resource into master

This is the second part of !412 (merged) (and does not depend on the first part).

The first dozen or so commits move the code around to unify resource v1, v2, and nop support. The last 3 commits improve resource cleanup for --multi-clients.

@gurchetansingh @gerddie @afrantzis

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