Fix: keep raster fade timer when reloading a tile that has data (#8106)

* Fix: keep raster fade timer when reloading a tile that has data

_tileLoaded reset timeAdded on every load, so reloading a tile that was
already on screen restarted its fade from zero opacity and briefly flashed
the map background. Reloads happen on setProjection, on expiry refresh and
on source data changes, so this showed up well outside of initial loading.

The fade is now re-based only when the tile held no data before the load,
which preserves the existing behaviour for edge tiles that latch a self
fade while still empty.

* Test: Make test name clearer

* Test: drive the reload fade tests through the public api

* Test: Trim tests

* Docs: Add comment to clarify what hadData guard does

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Co-authored-by: Frank Elsinga <frank@elsinga.de>