app: make the alpha channel work on canvas overlay widgets
* app/widgets/gimpoverlaychild.c: when creating offscreen windows, try to set an RGBA colormap on the offscreen window's widget and use the RGBA colormap to create the window. This has to be done here, because it's not possible to get to the right screen *inside* the offscreen widget before it's parented, and we need that screen before the widget is realized, and the widget can't be parented before it's realized or it will get the wrong parent window. Everything clear now? * app/widgets/gimpoverlayframe.c: draw the round corners only if the screen has an RGBA colormap.
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