Auto merge of #8291 - ecoal95:webgl-drop, r=jdm

Add destructors to some WebGL objects, remove duplicated glutin dependency and try to enable the webgl reftests

The first commit allows to cleanup the gl resources of the webgl task earlier if they aren't being used.
Right now all resources were cleaned up when the context was destroyed, so I think this is
a slightly better approach.

The second commit bumps rust-offscreen-rendering-context to remove the duplicated glutin dependency.

The third one tries to reenable the webgl reftests.
Since the errored builds are deleted, It's the only way I can try to troubleshoot it.

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