Move asset_compiler into its own (static) library.
This removes the dependency on it from the runtime and makes it possible
to have a standalone asset_compiler tool (useful for modding support).
Split meson.build into sub-files to improve readability.
Give each target its own pch directory.
This is more inline with what the meson manual recommends.
Export dependencies to make it possible to use the engine as a meson
subproject.
This is currently untested.
