Mesh::load_vtk() always open files in binary mode

[jdtournier]

Fix #2653.

[Lestropie]

Makes sense, and passes MSYS2 tests.
One small suggestion.
Should this instead (/ also) be going to master?

[jdtournier]

Should this instead (/ also) be going to master?

Yes, I think it will need to. I've only worried about dev so far since this is what's blocking the compiler caching changes to the CI (#2624), but there's no doubt a recent build of master on a recent version of MSYS2 will also be affected. What I'm not sure about is whether the precompiled binaries we provide for MSYS2 would also be affected when running on a modern version of MSYS2 - it depends on whether the bug is in the system C/C++ dynamic libraries that MRtrix links to at runtime, or in the MRtrix package code generated by the compiler...