Fix memory leak in the Cocoa backend.

This was reported here:
https://sft.its.cern.ch/jira/browse/ROOT-8951

Fix from Timur:

1. Add missing TGCocoa::ClosePixmap implementation. Apparently, while scanning for this function
    calls in TPad's code, I forgot to check TPadPainter and thus decided it's never used at all.
2. Make DestroyDrawable more 'atomic' - hide the fact that we first have to 'select drawable'
    (gVirtualX->SelectWindow(id) - note, SelectWindow for pixmap!!!) and then call
    ClosePixmap() - this is due to the old design problem/inconsistency in X11 backend
    and TPad::fPixmapID, which is not a real pixmap ID but some index that points to array
    where the real pixmap id is. Now DestroyDrawable accepts this fPixmapID and does
   all work of selecting pixmap and closing it.