Suggestion to change sphinx.ext.imgmath temporary base path to be based on out_dir

[jdknight]

Experienced some time ago when running on Windows and using dvisvgm from MiKTeX, image conversions when using sphinx.ext.imgmath would fail due to "Windows API error 87" failures. The issue appeared to be related to having Sphinx documentation being held on a different root drive (e.g. D:) over the temporary root drive (C:). Worked around this scenario for a builder by explicitly overriding the temporary path as follows (sphinx-contrib/confluencebuilder#756):

self._imgmath_tempdir = Path(tempfile.mkdtemp(prefix='.imgmath-', dir=self.out_dir))

It has been a moment since trying this recently, but noticed the Windows API error 87 error was showing itself again. Trying to look at why, it appears in #13626 that Sphinx dropped (unofficial) support of the _imgmath_tempdir override.

Wondering if it would be possible to update Sphinx to utilize the output directory to address this issue.