Adds a mir pixel format parameter to DisplayConfiguration::configure_output
So within DisplayConfigurationPolicy the format can be specified for each output.
The pixel format parameter is still ignored by android and also mesa to some degree,
which only validates it. It is supported by nested display. The default configuration
policy implementation tries to select an opaque format though. Fixes: https://bugs.launchpad.net/bugs/1256702.
Approved by PS Jenkins bot, Daniel van Vugt, Robert Carr.