[FEAT] Add the ability to hook into different mesa installations

[Samueru-sama]

Similar to how mesa snap interfaces work: https://api.snapcraft.io/api/v1/snaps/download/HyhSEBPv3vHsW6uOHkQR384NgI7S6zpj_1165.snap

Basically sharun would need to do what it already does but in a different lib directory.

This can be accomplished manually by using SHARUN_EXTRA_LIBRARY_PATH however there are some limitations:

• sharun will still be setting GBM_BACKENDS_PATH, LIBGL_DRIVERS_PATH and LIBVA_DRIVERS_PATH to its bundled mesa instead of the external installation.

• The order in which the vulkan drivers load will still be totally random.