windows {write|read}_mesh abs path round trip issue

[cyrush]

When save mesh is written using an abs path base on windows -- the following gets embedded in the root file:

D:\a\ascent\ascent\build\tests\_output/

This leads to a combined tree pattern that looks like:

D:\a\ascent\ascent\build\tests\_output/D:\a\ascent\ascent\build\tests\_output\tout_render_3d_mixed_thresh_extract.root"

which is wrong and triggers an exception on read:

IOHandle does not support opening paths with subpaths specified: "D:\a\ascent\ascent\build\tests\_output/D:\a\ascent\ascent\build\tests\_output\tout_render_3d_mixed_thresh_extract.root"

1. In this case, there output dir should be empty
2. Mixed use of \ and / is confusing.