Adds the swap chain interfaces to the API without changing the behavior
of anything else. This includes the C APIs for applications to provide
swap chain implementations. Also adds stub implementations on every
backend.
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.
