backends/drm: deep color support

This should reduce banding on outputs that support color formats with more
than 8 bits per color channel. In order to support this the commit also
removes the dependency of the EglGbmBackend on a specific format and
EglConfig and instead makes those per-Output properties.
2 jobs for !1641 with work/deepcolor in 21 minutes and 11 seconds (queued for 1 second)
detached
Status Name Job ID Coverage
  Build
passed freebsd_qt515 #165172
FreeBSD

00:06:48

passed suse_tumbleweed_qt515 #165171
Linux

00:21:11