wlr_texture_to_dmabuf involves issuing EGL commands to get a DMA-BUF out
of an EGLImage.
When the client submits a DMA-BUF via the linux-dmabuf protocol, there's
no need to go through EGL. We can directly import the client's DMA-BUF
into Vulkan.
The old code is still retained as a fallback in case the client uses
wl_drm (old Xwayland) or wl_shm. Removing it would involve creating a
Vulkan-based wlr_renderer.
References: https://github.com/Plagman/gamescope/issues/16
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.
