Support for variables (like @project_folder)

[SupaplexOSM]

I have a QGIS project that uses some variables, for example to specify relative file paths to image files for symbology (e.g. define an image source as @project_folder + '/images/symbol.svg') or to increase line widths by a specified, freely adjustable factor (e.g. define the width of a line as 1.5 * @scale_factor).

If I render this project with QTiles, these variables don't seem to be readable. For example, image files are not loaded and line width aren't adjusted. If I replace the variables, however, it works without errors.

(Using 3.22.4-Białowieża with QTiles 1.7.2 on Ubuntu 22.04.)