util/transform: move over wl_output_transform helpers

These aren't really tied to wlr_output.

include/wlr/types/wlr_output.h

@@ -729,19 +729,4 @@ bool wlr_output_configure_primary_swapchain(struct wlr_output *output,
 struct wlr_render_pass *wlr_output_begin_render_pass(struct wlr_output *output,
 	struct wlr_output_state *state, int *buffer_age, struct wlr_render_timer *timer);
 
-
-/**
- * Returns the transform that, when composed with `tr`, gives
- * `WL_OUTPUT_TRANSFORM_NORMAL`.
- */
-enum wl_output_transform wlr_output_transform_invert(
-	enum wl_output_transform tr);
-
-/**
- * Returns a transform that, when applied, has the same effect as applying
- * sequentially `tr_a` and `tr_b`.
- */
-enum wl_output_transform wlr_output_transform_compose(
-	enum wl_output_transform tr_a, enum wl_output_transform tr_b);
-
 #endif

include/wlr/util/transform.h

@@ -0,0 +1,28 @@
+/*
+ * This an unstable interface of wlroots. No guarantees are made regarding the
+ * future consistency of this API.
+ */
+#ifndef WLR_USE_UNSTABLE
+#error "Add -DWLR_USE_UNSTABLE to enable unstable wlroots features"
+#endif
+
+#ifndef WLR_UTIL_TRANSFORM_H
+#define WLR_UTIL_TRANSFORM_H
+
+#include <wayland-server-protocol.h>
+
+/**
+ * Returns the transform that, when composed with `tr`, gives
+ * `WL_OUTPUT_TRANSFORM_NORMAL`.
+ */
+enum wl_output_transform wlr_output_transform_invert(
+	enum wl_output_transform tr);
+
+/**
+ * Returns a transform that, when applied, has the same effect as applying
+ * sequentially `tr_a` and `tr_b`.
+ */
+enum wl_output_transform wlr_output_transform_compose(
+	enum wl_output_transform tr_a, enum wl_output_transform tr_b);
+
+#endif