BLI: Overload float4x4 multiplication-assignment operator

This looks a lot nicer than writing `mul_m4_m4_post` instead.

Differential Revision: https://developer.blender.org/D12844

source/blender/blenlib/BLI_float4x4.hh

@@ -124,6 +124,11 @@ struct float4x4 {
     return result;
   }
 
+  void operator*=(const float4x4 &other)
+  {
+    mul_m4_m4_post(values, other.values);
+  }
+
   /**
    * This also applies the translation on the vector. Use `m.ref_3x3() * v` if that is not
    * intended.