For a quick tiled window we currently don't draw a border on the edges it is
tiled to but we still draw a shadow. In a one screen case this is cannot be
noticed because the shadows are outside of the screen. In a multi screen
environment however this causes quick tiled windows to cast shadow across the
screen border that they are tiled to. If one has multiple of such tiled windows
it darkens the neighboring area of the bordering screen significantly.
This patch uses the same criteria whether to draw a shadow on a side as is used
to determine if a border should be drawn. Each decoration instance now has its
own shadow instance. The actual shadow texture is still only generated once and
shared, the partial shadows are partial copies of it excluding the parts of the image
that should not be rendered. For example if a window is tiled to the top left corner
the cropped shadow texture will not include the top and left part as well as not the
top left, top right and bottom left corner.