[Proton#4288] Gets broken by host graphics libraries in /opt

In https://github.com/ValveSoftware/Proton/issues/4288 the user had a copy of libdrm.so.2 in /opt, part of a third-party OpenCL driver (amdgpu-pro) which somehow added it to the global search path for shared libraries (presumably by either dropping a file into /etc/ld.so.conf.d/ or editing /etc/ld.so.conf).

When pressure-vessel captures the host graphics stack, the user's GLX driver depends on libdrm.so.2, so we "capture" libdrm.so.2 and put a symlink to it in /overrides.

However, there's a problem:

• The symlink is to /run/host/opt
• There is no /run/host/opt
• There is no /opt either

We could mount /opt on /run/host/opt, but that would break the Flatpak use-case. What we probably need to do is:

• Replace capsule-capture-libs --link-dest=/run/host with something like capsule-capture-libs --remap-link-dest=/usr=/run/host/usr --remap-link-dest='/lib*=/run/host/lib*'
• For each library that is found to be outside the /usr-like directories, do the equivalent of --filesystem=/opt/path/to/driver/libdrm.so.2:ro

That would also allow us to deal with graphics libraries in $HOME (I think).