wayland: shm based software rendering

A wayland output based on shared memory. This video output is useful for x11
free systems, because the current libGL in mesa provides GLX symbols. It is also
useful for embedded systems where the wayland backend for EGL is not
implemented like the raspberry pi.

At the moment only rgb formats are supported, because there is still no
compositor which supports planar formats like yuv420p. The most used compositor
at the moment, weston, supports only BGR0, BGRA and BGR16 (565).

The BGR16 format is the fastest to convert and render without any noticeable
differences to the BGR32 formats. For this reason the current (very basic)
auto-detection code will prefer the BGR16 format. Also the weston source code
indicates that the preferred format is BGR16 (RGB565).

There are 2 options:
    * default-format (yes|no)  Which uses the BGR32 format
    * alpha (yes|no)    For outputting images and videos with transparencies