Fix vtkAxis scaling when TileScale is set on the renwin.
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.
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