Threaded AV doesn't update label sizes in AV tree

[zbynek]

With

GuiManager::USE_COMPRESSED_VIEW = true;

create function x² and drag it so that the formula becomes longer -- you will only see part of it. Doesn't happen with

GuiManager::USE_COMPRESSED_VIEW = false;

[michael]

also setting private static int CV_UPDATES_PER_SECOND = 1; and dragging x^{2 doesn't seem to slow down the update in the AV}

[lucas]

[14081] should fix the described problem

please test