Can we get a custom shader path flag or env var?

[adigitoleo]

Building from the development branch, without copying the shaders to the hardcoded /usr/share/sioyek/shaders, causes sioyek to silently render blank pages for every document. Can we get the following:

• a way to set a custom shader path either with a flag or SIOYEK_SHADERS variable
• an error message and exit with code 1 if there are no shaders at that path

This will maybe prevent more "why are there blank pages" reports down the line and will help with debugging.

Cheers.