perf: Double rendering

[mmeeks]

Open a spreadsheet, hit page-down a few times - notice some tiles have been rendered twice.

We should save that double rendering. I believe this is because we send a tilecombine that is interpreted as being a request to force rendering of a tile - and the JS client shoudl not have that 'power' - it should be sent a cached version.

Meanwhile DocumentBroker needs a way to force a new key-frame, and I tihnk the JS does this by accident, and sometimes these race unhelpfully.

So the 'oldwid=' thing needs adapting / filtering from the JS side to avoid this in the Kit/ code =)

Thanks !

[mmeeks]

You can see the count of tile rendering in the grid overlay over the tiles with details there.