Zooming in to imagery shows degraded tiles

[timlinux]

This seems to be an artifact of the tile rendering chain:

With WMS versus XYZ and QGIS Backend:

Tiles should render at the same resolution regardless of whether XYZ tiles or WMS tiles are used.

child of #457

[gubuntu]

@NyakudyaA low priority: see if you can get to the bottom of this after confirming it's still the case (remember do all testing on the testing server)

[gubuntu]

still the case @NyakudyaA ?