feat: remove reliance on compile-time env variables

[imgurbot12]

Addresses Issue: https://github.com/RobinMcCorkell/shaderlock/issues/1

Removes reliance on compile-time environment variables and instead allows them to be set during runtime. If the variables are not set, it uses sane defaults based on the previous configuration as well as whats defined according to the X Freedesktop Standard Specification.