[shell] Add Wayland support to PanelShadow
Review Request #124360 - Created July 15, 2015 and submitted
This change adds support for shadows using the KWin specific shadow protocol provided by KWayland. The protocol is inspired by the X11 protocol, so it can be easily integrated. The protocol takes a buffer for each of the shadow parts and provides the same offsets. To make this possible the code is better split into X11 and Wayland specific parts.