This allows proper integration with logind/seatd. When switching
VTs, gamescope now dies with ENOPERM instead of making VT switching
freeze.
References: https://github.com/Plagman/gamescope/issues/6
This avoids needlessly creating buffers for an headless output we won't
use, and will allow us to more easily replace the headless backend with
a custom renderer in the future.
wlroots headers can mostly be included with extern "C" blocks. Two
exceptions are the use of `static` for array args and `class` as a
struct field. These can be #define'ed to avoid C++ build errors.
This allows the whole project to be built as a single C++ codebase,
without having to maintain hybrid header files.
