Add full DRM/KMS backend with libseat session, GBM allocation, EGL
display initialisation, and a GpuManager-driven rendering loop.
- drm_device.rs: type aliases and per-device/per-output state structs
(WeftDrmDevice, WeftOutputSurface, WeftDrmData)
- drm.rs: replace skeleton with complete backend libseat session,
udev device enumeration, libinput event source, connector scanning
via smithay-drm-extras DrmScanner, DrmOutputManager initialisation
per CRTC, VBlank-driven render_output, sd_notify(READY=1)
- state.rs: add drm: Option<WeftDrmData> field; route dmabuf import
through GPU manager when the DRM path is active
- Cargo.toml: add renderer_multi, use_system_lib Smithay features;
add smithay-drm-extras and sd-notify Linux dependencies
render_output submits a clear-colour-only frame to establish
the VBlank pipeline. Surface compositing is wired up in a subsequent commit.
