add frame duplicate

src/rtt.cpp

@@ -174,6 +174,14 @@ void RTT::copy(const RTT& source, const glm::vec4& rect)
     });
 }
 
+RTT RTT::clone() const noexcept
+{
+    RTT dup;
+    dup.create(w, h);
+    dup.copy(*this);
+    return dup;
+}
+
 bool RTT::create(int width, int height, int tex/* = -1*/, GLint internal_format, bool depth_buffer /*= false*/)
 {
     GLenum status = 0;