DebugTilesPlugin: improve "init extremes" performance

[gkjohnson]

When a new child tileset is loaded the whole hierarchy is loaded which can take 40+ ms in some cases especially with complex tilesets like google tiles, causing a jerkiness when new tiles are loaded in.

Options:

• Derive extremes only from currently visible tilesets
• Take the max of the current extreme and any newly added tileset

[sguimmara]

I didn't see this issue, but it might be obsolete now with #885

[gkjohnson]

I think it is!