perf: de-prioritize rendering non-visible tiles

[sgothel]

• Resolves: #9989
• Target version: master

Summary

Only render invisible combined-tiles if IDLE

As an attempt to increase responsiveness for client, we only send invisible requested combined-tiles if IDLE. IDLE is determined if socket poll timed out, i.e. nothing else to do for client.

Checklist

• [ ] I have run make prettier-write and formatted the code.
• [ ] All commits have Change-Id
• [ ] I have run tests with make check
• [ ] I have issued make run and manually verified that everything looks okay
• [ ] Documentation (manuals or wiki) has been updated or is not required