Complete the cinema path timescale implementation

This patch fixes the previously broken implementation of the timescale
property of cinema paths, now cinema paths with timescale 0.5, for
example, play twice as fast.
Unlike the comment stated, negative timescales weren't actually
supported, which is now enforced.
For demonstration, the timescale of the demo map's path is changed to
0.85.

binaries/data/mods/public/maps/scenarios/cinema_demo.xml

@@ -14063,7 +14063,7 @@
 		</Entity>
 	</Entities>
 	<Paths>
-		<Path name="test" timescale="1" orientation="target" mode="ease_inout" style="default">
+		<Path name="test" timescale="0.85" orientation="target" mode="ease_inout" style="default">
 			<Node deltatime="0">
 				<Target x="450.11701" y="15" z="910.41565"/>
 				<Position x="467.2" y="40.87" z="887.26"/>