Fixes: NB#216314 - With attached xserver fixes for CompositeUnredirectWindow global alpha is set incorrectly - move global alpha setting to checkStacking() to make it stacking order   sensitive - only one global alpha window is considered at a time
Fixes: NB#216314 - With attached xserver fixes for CompositeUnredirectWindow global alpha is set incorrectly
- move global alpha setting to checkStacking() to make it stacking order
  sensitive
- only one global alpha window is considered at a time

nemomobile-graveyard/mcompositor,siraj/mcompositor,nemomobile-graveyard/mcompositor,nemomobile-graveyard/mcompositor,siraj/mcompositor,nemomobile-graveyard/mcompositor,siraj/mcompositor,nemomobile-graveyard/mcompositor,siraj/mcompositor,nemomobile-graveyard/mcompositor