Keep track of how many tileload/tileunload events are being fired. The counts should always match. See #4093, #4193

| Start | Load | Error | Unload | Visible | Grid load
--- | --- | --- | --- | --- | --- | ---
Grid |  |  |  |  |  |
Positron |  |  |  |  |  |

start = unload + visible on screen

(flyTo)
(setView, 5 sec)
(flyTo 20 sec)
(fract. zoom)
(setView, fract. zoom)
(fitBounds)
(image overlay)