Replace maven.multiModuleProjectDirectory with openrs2.root

openrs2.root works correctly even if the root of the reactor is a child
project, at the expense of having to re-define the property in each
module.

gl-dri-natives/pom.xml

@@ -13,6 +13,10 @@
 
   <name>OpenRS2 GL DRI Natives</name>
 
+  <properties>
+    <openrs2.root>${project.parent.basedir}</openrs2.root>
+  </properties>
+
   <profiles>
     <profile>
       <id>unix</id>

gl-natives/pom.xml

@@ -13,6 +13,10 @@
 
   <name>OpenRS2 GL Natives</name>
 
+  <properties>
+    <openrs2.root>${project.parent.basedir}</openrs2.root>
+  </properties>
+
   <build>
     <plugins>
       <plugin>

misc-natives/pom.xml

@@ -12,4 +12,8 @@
   <packaging>nar</packaging>
 
   <name>OpenRS2 Misc Natives</name>
+
+  <properties>
+    <openrs2.root>${project.parent.basedir}</openrs2.root>
+  </properties>
 </project>