WM: various changes to file writing behavior

Saving with only a filename (from Python) wasn't being prevented,
while it would successfully write the file to the working-directory,
path remapping and setting relative paths wouldn't work afterwards
as `Main.filepath` would have no directory component.

Disallow this since it's a corner case which only ever occurs
when path names without any directories are used from Python,
the overhead of expanding the working-directory for all data saving
operations isn't worthwhile.

The following changes have been made:

- bpy.ops.wm.save_mainfile() without a filepath argument
  fails & reports and error when the file hasn't been saved.

  Previously it would write to "untitled.blend" and set the
  `G.main->filepath` to this as well.

- bpy.ops.wm.save_mainfile(filepath="untitled.blend")
  fails & reports and error as the filename has no directory component.

- `BLI_path_is_abs_from_cwd` was added to check if the path would
  attempt to expand to the CWD.

1 files changed, 1 insertions, 0 deletions

diff --git a/source/blender/blenloader/intern/writefile.c b/source/blender/blenloader/intern/writefile.c
index e0ff70d4b49..aa3eef4b475 100644
--- a/source/blender/blenloader/intern/writefile.c
+++ b/source/blender/blenloader/intern/writefile.c
@@ -1319,6 +1319,7 @@ bool BLO_write_file(Main *mainvar,
                     ReportList *reports)
 {
   BLI_assert(!BLI_path_is_rel(filepath));
+  BLI_assert(BLI_path_is_abs_from_cwd(filepath));
 
   char tempname[FILE_MAX + 1];
   WriteWrap ww;