Keep the texture picker redrawing while thumbnails load

Marking the canvas dirty from inside the render does not schedule another
idle pass, so deferred thumbnails waited for the next mouse event. Asking
for an extra frame does.

src/slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.cpp

@@ -4364,7 +4364,7 @@ void GLGizmoTextureDisplacement::render_texture_library_popup(const ImVec2 &pane
             // The selected tile is centred even when it is off screen, so it is drawn and loaded next frame.
             if (current && m_picker_reveal_selected) {
                 ImGui::SetScrollFromPosY(0.5f * (rect_min.y + rect_max.y) - ImGui::GetWindowPos().y, 0.5f);
-                m_parent.set_as_dirty();
+                m_parent.request_extra_frame();
             }
             if (!ImGui::IsRectVisible(rect_min, rect_max))
                 continue;
@@ -4597,7 +4597,7 @@ void GLGizmoTextureDisplacement::render_texture_library_popup(const ImVec2 &pane
         ImGui::PopTextWrapPos();
     }
     if (loads_deferred)
-        m_parent.set_as_dirty();
+        m_parent.request_extra_frame();
     // Done whether or not the current texture was drawn: one in a collapsed section stays where it is.
     m_picker_reveal_selected = false;
     ImGui::EndChild();