Otherwise we would end up with rendering artifacts due to overpainting

…tions

Yes, that means OpenStreetMap tiles and various other sources!

No GUI so far...

... so we first receive tiles in the middle rather those at the border

Marked raster interface as uncopiable (it should have always been uncopiable)

Moved all temporary projector members to a private class,
so even recursive block() calls will not affect each other
(there is no new performance penalty as block() call always
recomputes the temporary control point matrix anyway)

It would make sense to have it enabled by default with WMTS too

This makes it easier to get at least basic preview without having to wait
until new tiles are downloaded. The strategy is to use tiles up to two levels
lower resolution tiles and one level higher resolution tiles.

Max. 100 tiles is just too low. The max request area is currently 2000x2000 pixels
which boils down to approx 8x8 tiles of size 256x256, therefore max 64 tiles.
However this is the ideal case if the view scale matches the scale of tiles.
If the map view scale is approx in between two levels, we may need 12x12 tiles
to serve the request, so bumping the maximum to 256 should be enough.