Line Symbol offset can't be negative when using a CRS with degree units and 'Meters at Scale'

[lawrence-n]

What is the bug or the crash?

When using a CRS with degree units, attempting to use a negative offset with 'Meters at Scale' units will draw at the positive offset. This bug affects various symbol layer types for lines and polygons.

I think this is because QgsRenderContext::convertMetersToMapUnits will always return a positive value when the CRS uses degrees.

Steps to reproduce the issue

1. Use a line symbol with a negative offset using 'Meters at Scale' units.
2. Change the project's CRS between coordinate systems measured in metres and degrees.
3. Note that the symbol layer with the negative offset will switch sides when the CRS changes.

Versions

QGIS version
3.20.0-Odense
QGIS code revision
decaadb
Qt version
5.15.2
Python version
3.9.5
GDAL/OGR version
3.3.0
PROJ version
8.0.1
EPSG Registry database version
v10.018 (2021-04-02)
GEOS version
3.9.1-CAPI-1.14.2
SQLite version
3.35.2
PDAL version
2.3.0
PostgreSQL client version
13.0
SpatiaLite version
5.0.1
QWT version
6.1.3
QScintilla2 version
2.11.5
OS version
Windows 10 Version 2009

Supported QGIS version

• I'm running a supported QGIS version according to the roadmap.

New profile

• I tried with a new QGIS profile

Additional context

No response

[gioman]

3.20.0-Odense
Supported QGIS version

@lawrence-n maybe won't change anything but better test with a recent release, 3.20.0 is already behind many bug fixes.