quaternions: Allow multiplication into 'a' arg

When multiplying two quaternions, we now implicitly copy the components
of the 'a' argument so that the result can be reliably written back to
the 'a' argument quaternion without conflicting with the multiplication
itself. This is consistent with the cogl_matrix_multiply() api which
allows the 'result' and 'a' arguments to point to the same matrix. In
debug builds Cogl will assert that the 'b' and 'result' arguments don't
point to the same quaternion.

Reviewed-by: Neil Roberts <neil@linux.intel.com>

(cherry picked from commit 207527313a8957789390069e84189254cf41e88f)

cogl/cogl-quaternion.c

@@ -434,11 +434,18 @@ cogl_quaternion_multiply (CoglQuaternion *result,
                           const CoglQuaternion *a,
                           const CoglQuaternion *b)
 {
-  result->w = a->w * b->w - a->x * b->x - a->y * b->y - a->z * b->z;
+  float w = a->w;
+  float x = a->x;
+  float y = a->y;
+  float z = a->z;
 
-  result->x = a->w * b->x + a->x * b->w + a->y * b->z - a->z * b->y;
-  result->y = a->w * b->y + a->y * b->w + a->z * b->x - a->x * b->z;
-  result->z = a->w * b->z + a->z * b->w + a->x * b->y - a->y * b->x;
+  _COGL_RETURN_IF_FAIL (b != result);
+
+  result->w = w * b->w - x * b->x - y * b->y - z * b->z;
+
+  result->x = w * b->x + x * b->w + y * b->z - z * b->y;
+  result->y = w * b->y + y * b->w + z * b->x - x * b->z;
+  result->z = w * b->z + z * b->w + x * b->y - y * b->x;
 }
 
 void

cogl/cogl-quaternion.h

@@ -394,6 +394,9 @@ cogl_quaternion_invert (CoglQuaternion *quaternion);
  * so the rotations are applied @right to @left. This is similar to the
  * combining of matrices.
  *
+ * <note>It is possible to multiply the @a quaternion in-place, so
+ * @result can be equal to @a but can't be equal to @b.</note>
+ *
  * Since: 2.0
  */
 void