backend/drm: implement KMS state snapshot/restore

Following ideas from [1], snapshot the entire KMS state when the VT
is switched away, and restore it when the VT is switched back.

> Well the neat trick is that userspace doesn’t need to be able to
> understand properties to save and restore them - the actual property
> value transport between kernel and userspace is fully generic.

That way, even if another DRM master changes a property we don't
understand like CTM or HDR_OUTPUT_METADATA, we can switch it back
and avoid getting garbage on screen.

[1]: https://blog.ffwll.ch/2016/01/vt-switching-with-atomic-modeset.html

include/backend/drm/drm.h

@@ -85,6 +85,8 @@ struct wlr_drm_backend {
 	uint64_t cursor_width, cursor_height;
 
 	struct wlr_drm_format_set mgpu_formats;
+
+	drmModeAtomicReq *atomic_snapshot;
 };
 
 struct wlr_drm_mode {

include/backend/drm/iface.h

@@ -17,6 +17,8 @@ struct wlr_drm_interface {
 	bool (*crtc_commit)(struct wlr_drm_connector *conn,
 		const struct wlr_drm_connector_state *state, uint32_t flags,
 		bool test_only);
+	bool (*snapshot_state)(struct wlr_drm_backend *drm);
+	bool (*restore_state)(struct wlr_drm_backend *drm);
 };
 
 extern const struct wlr_drm_interface atomic_iface;
@@ -24,5 +26,6 @@ extern const struct wlr_drm_interface legacy_iface;
 
 bool drm_legacy_crtc_set_gamma(struct wlr_drm_backend *drm,
 	struct wlr_drm_crtc *crtc, size_t size, uint16_t *lut);
+bool legacy_restore_state(struct wlr_drm_backend *drm);
 
 #endif

include/backend/drm/util.h

@@ -16,6 +16,8 @@ const char *get_pnp_manufacturer(uint16_t code);
 void parse_edid(struct wlr_drm_connector *conn, size_t len, const uint8_t *data);
 const char *drm_connector_status_str(drmModeConnection status);
 
+drmModeAtomicReq *snapshot_drm_state(int drm_fd);
+
 // Part of match_obj
 enum {
 	UNMATCHED = (uint32_t)-1,