Now vtkAxis will show the same number of ticks and the
same relative text size when magnifying the image. Text
will still be a little funky if the scaling is anisotropic,
but at the moment our freetype engine doesn't support
anisotropic dpi (not sure if the MPL backend can or not).
New API: vtkContextScene::GetLogicalTileScale() returns
the renderwindow tilescale or (1, 1) depending on whether
ScaleTiles is enabled.
There are some random fixes where vtkAxis objects were being
used without a scene, which led to segfaults when trying to
look up the tile scale internally.