Add "dedicated server" export mode which can strip unneeded visual resources

doc/classes/Cubemap.xml

@@ -11,4 +11,12 @@
 	</description>
 	<tutorials>
 	</tutorials>
+	<methods>
+		<method name="create_placeholder" qualifiers="const">
+			<return type="Resource" />
+			<description>
+				Creates a placeholder version of this resource ([PlaceholderCubemap]).
+			</description>
+		</method>
+	</methods>
 </class>