This is meant for a "permanent" tear-down before freeing the memory
holding the VM state or at program shutdown. As a consequence, builtin
sub-systems registering resources are now required to pass a "destroy"
function pointer that will be called just before the memory holding
those resources is freed by the VM resource manager (ie, the manager
owns the resource memory block, but each subsystem is responsible for
cleaning up any resources held within that block).
This even enhances thread-safety in rua_obj (there are some problems
with cmd, cvar, and gib).
Just "loading" and "unloading" (both really just hints due to the
caching system), and an internal function for converting a handle to a
model pointer, but it let me test IQM loading and unloading in Vulkan.
