Detect format of client SHM buffer and render appropriately

[malcolmstill]

Currently ulubis assumes the format of an incoming buffer in the commit request. This means that, for example, the colour of the video coming from Cheese renders in the wrong colours.

[malcolmstill]

So Wayland is (typically) giving buffers that are in ARGB (which due to endianess becomes BGRA) format whereas we're telling OpenGL the format is RGBA therefore we need to swap the red and green components.

[malcolmstill]

Fixed by d6ffa55