chore(context-pad): update position once per frame

[nikku]

Proposed Changes

As shown in this flame graph, in a huge diagram, we spend substantial amount of time to re-compute canvas viewport (after select) and determine context pad position (to update it):

All this happens within the user transaction, and blocks UI interactivity.

With this PR we batch position updates as we batch visibility updates; we'll otherwise force the browser into expensive in-frame repaints:

Related to https://github.com/bpmn-io/diagram-js/pull/915, https://github.com/camunda/camunda-modeler/issues/4335.

Checklist

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• [x] Brief textual description of the changes present
• [x] Visual demo attached
• [ ] Steps to try out present, i.e. using the @bpmn-io/sr tool
• [x] Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}

[marstamm]

Unrelated to this PR, but it seems like the scheduler tests consistently fail on CI Linux

[nikku]

Follow-up to the Firefox test failures: https://github.com/bpmn-io/diagram-js/issues/921.