monitor-config-manager: Support logical monitor transforms

Add support for rotated monitors. This is done per logical monitor, as
every monitor assigned to a logical monitor must be transformed in the
same way. This includes being transformed on the same level; e.g. if
the backend does not support transforming any monitor of a logical
monitor natively, then all monitors will be transformed using the
offscreen intermediate framebuffer.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/meta-monitor-manager.c

@@ -1736,6 +1736,7 @@ create_logical_monitor_config_from_variant (MetaMonitorManager          *manager
       .width = width,
       .height = height
     },
+    .transform = META_MONITOR_TRANSFORM_NORMAL,
     .scale = (int) scale,
     .is_primary = is_primary,
     .monitor_configs = monitor_configs