Code review comment for lp:~andreas-pokorny/mir/allow-transparent-server-buffers

Choosing the pixel format of each output's framebuffer is part of the display (re)configuration process, so I think that it conceptually belongs in DisplayConfigurationPolicy. However, I don't think that any changes to the policy interface are actually needed. We only need to allow the policy to set the format, by extending DisplayConfiguration::configure_output() to accept a 'format_index' parameter (note that DisplayConfigurationOutput already contains a 'pixel_formats' and 'current_format_index' fields).