#150 Allow skewing/offset for WMTS etc. layers

[jidanni]

Please allow skewing/offset for WMTS etc. layers as described in
https://sourceforge.net/p/viking/discussion/general/thread/cfcf7c8c0f/

[Rob Norris]

Would a single pair of offset values (applying at all zoom levels) be good enough?

Otherwise implementing varying values per zoom level requires much more work.
i.e. Needs a rethink on how that would work within the current format of the maps.xml file
(if stored there rather than in the map layer properties that gets saved in .vik files);
how the gui interface would show it (if at all);
or only via moving the map itself...

I'm not sure I have the inclination to do anything too onerous for something I personally wouldn't use (vs all the other things I'm doing).

[jidanni]

Sure, fine!
I'm just trying to move a layer a few meters north and east for now.