Calculation precision overhaul for Maps

In Map and Projection and friends, we use QVector3D extensively for performing calculations relating to coordinate translation and projection. As QVector3D and friends use 32-bit floats on all platforms (not qreals), this opens up a can of precision worms.

Some issues (eg the panning at high zoom) may be possible to patch individually in a temporary fashion, but in the long-run, a full cleanup of precision-related issues is probably going to be necessary.