UI: Fix sizing of framebuffer attachments so that they are fully visible and can be zoomed and scrolled.
* Also rearranged code so that the layout of the attachments is only resized in slot_zoomFactorChanged(..) rather than also when the widgets are initially added.
* Add a flag to delay the handling of ZoomFactorChanged so that the window is NOT resized each time an attachment is added, but only after they are all added.
* I can't find what caused this to stop working, but the GroupBoxes which contain each of the attachments needed to be resized so that all the attachments has sufficient room to be displayed. The height also needed to be increased slightly to accomodate for the horizontal scrollbar if the texture becomes wider than the widget.
Merge branch 'master' of https://github.com/ValveSoftware/vogl