Applying the "dim window" effect to the MetaWindowActor has two avantages:
first it avoids triggering bugs where ClutterOffscreenEffect doesn't handle
clone paint correctly. Second, it avoids showing the window as dimmed in
alt-Tab and the overview, which is weird.

The small downside of this is that the shadow becomes slightly gray when
the window dimmed, which is wrong - if we switched from blending with gray
to a combination of desaturation and darkening, this problem wouldn't
happen.

Revert out the addition of startY to the shader, since we don't need it
and fix the application of alpha, since we need to handle alpha correctly
for the shadow.