Register reusable platform and VTK conditional skip markers

[banesullivan]

Downstream PyVista projects keep reinventing skip_egl, skip_mac, skip_windows, and needs_vtk_version, or fall back to raw skipIf/importorskip that is harder to read and easy to get wrong. This registers all four markers and their pytest_runtest_setup evaluation in the plugin.

needs_vtk_version accepts the positional form (needs_vtk_version(9, 3) meaning at least 9.3) and explicit at_least=/less_than= bounds, pads version tuples so (9, 3) compares correctly against (9, 3, 0), and raises a clear UsageError on a malformed marker. EGL detection falls back to running the test if PyVista's internal gl_checks module is unavailable on an older release.

The markers are opt-in by nature: the hook only acts on tests that carry them.

resolves #275

[user27182]

I'd personally rather see needs_vtk_version kept inside pyvista, and moved from its test suite into core, maybe _vtk_utilities.

Reason being that I'd like to move pv.vtk_version_info checks from inside filters that require newer VTK versions to use a decorator instead

@_needs_vtk_version(9, 3, 0)
def contour_labels(

So basically it can be used for runtime checks, or adapted for testing.

I made a commit a few days ago to start this but didn't get far pyvista/pyvista#8684