Implement a shader baking system

[stuarthayhurst]

After porting to Vulkan, bake the current graphics settings into the shaders, where possible. Keep a table of loaded shaders, and when graphics settings change, search the table first, then the disk cache and finally compile if neither of these match. The graphics settings will need to be included in the cache string (reuse functions from cache / file manager).

The post-processing shaders might heavily benefit from this after more options are introduced.