VTK doesn't seem produce png symbols on aarch64. We could probably
figure out how to fix that but we can also just tell it to use the
libpng we already have, which already works on aarch64.
vtk: clean up / reduce code repetition
This refactors the vtk expressions to use a generic base expression,
which is used for all 3 versions.
vtk 7.x no longer uses gcc 8, since the quirk of it not building with
gcc 9 seems to have been fixed.
This also makes the python bindings available for all 3 versions, and
fixes building them for vtk 8 by adding a patch.
