PipelineLayout was re-entering in NXT to create the default empty BGLs,
but forgot to remove the initial external refcount for them.
Fixing this showed an issue where it was impossible to externally
reference a RefCounted that had only internal references, fix this as
well and add a test.
This directory used to contain both the state tracking code for the
backends, and the common utilities that could be used both by the
backends and the rest of the code. Things are now:
- src/common is utility code for the whole repo
- src/backend contains libNXT's code
- src/utils is utility code that we don't want in libNXT
This commit also changes all includes to use global paths from src/
bacause it had to touch a bunch of #include statements anyway.
