monitors: enable libcamera by default and set priority on all camera nodes

Currently v4l2 nodes will get higher priority than libcamera ones.
The default can be changed, of course, with wpctl.

Things are still not great if a camera is enumerated by both monitors.
The first node to be linked will grab the device and the second one
from the other subsystem will fail to negotiate.

src/scripts/monitors/libcamera.lua

@@ -86,6 +86,20 @@ function createNode(parent, id, type, factory, properties)
   -- sanitize description, replace ':' with ' '
   properties["node.description"] = desc:gsub("(:)", " ")
 
+  -- set priority
+  if not properties["priority.session"] then
+    local location = properties["api.libcamera.location"]
+    local priority = 700
+    if location == "front" then
+      priority = priority + 150
+    elseif location == "external" then
+      priority = priority + 100
+    elseif location == "back" then
+      priority = priority + 50
+    end
+    properties["priority.session"] = priority
+  end
+
   -- apply properties from config.rules
   rulesApplyProperties(properties)