CoglMatrix: Don't make the matrix fields private

The xx, yx, zx etc fields are meant to be read-only but they were
marked as private with the gtk-doc annotation. This patch moves the
private marker so that the 16 float member fields are public but the
type, inverted matrix, flags and padding are not.

cogl/cogl-matrix.h

@@ -72,8 +72,6 @@ typedef struct _CoglMatrix      CoglMatrix;
  */
 struct _CoglMatrix
 {
-  /*< private >*/
-
   /* column 0 */
   float xx;
   float yx;
@@ -98,6 +96,8 @@ struct _CoglMatrix
   float zw;
   float ww;
 
+  /*< private >*/
+
   /* Note: we may want to extend this later with private flags
    * and a cache of the inverse transform matrix. */
   float   inv[16];