Ensure the file path G.main->filepath is always absolute and normalized.
- It was possible to call WM_OT_open_mainfile with only a filename,
if this resolved from the CWD, Blender's internal filepath
would not be absolute as expected.
- It was possible to open files on UNIX with an additional forward slash
causing the blend file path it's self to contain a '//' prefix,
this is error prone as running BLI_path_abs(..) multiple times would
add the blend file prefix each time.
- Remove requirement for "filepath" to be an absolute path when saving.
Instead, expand the path - making it absolute, as this constraint
wasn't applied open opening files, prefer making save/open behave
consistently.
- Assert when BLI_path_abs/BLI_path_rel receive a basepath that has
a "//" (relative) prefix itself.
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