matrix: move to util/

wlr_matrix is not a standalone type like other headers in types/,
it's more of an internal utility. Move it to the appropriate place.

include/util/matrix.h

@@ -0,0 +1,42 @@
+#ifndef UTIL_MATRIX_H
+#define UTIL_MATRIX_H
+
+#include <wayland-server-protocol.h>
+
+struct wlr_box;
+
+/** Writes the identity matrix into mat */
+void wlr_matrix_identity(float mat[static 9]);
+
+/** mat ← a × b */
+void wlr_matrix_multiply(float mat[static 9], const float a[static 9],
+	const float b[static 9]);
+
+/** Writes a 2D translation matrix to mat of magnitude (x, y) */
+void wlr_matrix_translate(float mat[static 9], float x, float y);
+
+/** Writes a 2D scale matrix to mat of magnitude (x, y) */
+void wlr_matrix_scale(float mat[static 9], float x, float y);
+
+/** Writes a transformation matrix which applies the specified
+ *  wl_output_transform to mat */
+void wlr_matrix_transform(float mat[static 9],
+	enum wl_output_transform transform);
+
+/** Shortcut for the various matrix operations involved in projecting the
+ *  specified wlr_box onto a given orthographic projection with a given
+ *  rotation. The result is written to mat, which can be applied to each
+ *  coordinate of the box to get a new coordinate from [-1,1]. */
+void wlr_matrix_project_box(float mat[static 9], const struct wlr_box *box,
+	enum wl_output_transform transform, const float projection[static 9]);
+
+/**
+ * Writes a 2D orthographic projection matrix to mat of (width, height) with a
+ * specified wl_output_transform.
+ *
+ * Equivalent to glOrtho(0, width, 0, height, 1, -1) with the transform applied.
+ */
+void matrix_projection(float mat[static 9], int width, int height,
+	enum wl_output_transform transform);
+
+#endif