Prevent alpha underflow (bug #14503)

Shadows - like most ghost units - have varying alpha levels as part of their animation, however, the Shadow also has the Nightstalker ability which reduces alpha by 0.5 (invisible unit).
As a result, there is an alpha underflow when the animation frames vary from -0.1 to 0.3.
By using a multiplication instead of an addition-1, as suggested by CelticMinstrel, the alpha variation becomes 0.2 to 0.4, avoiding the underflow and making the Shadow slightly less translucent at night.

src/units/frame.cpp

@@ -888,7 +888,7 @@ const frame_parameters unit_frame::merge_parameters(int current_time, const fram
 	/** The engine provides a highlight ratio for selected units and visible "invisible" units */
 	result.highlight_ratio = current_val.highlight_ratio != 1.0 ? current_val.highlight_ratio : animation_val.highlight_ratio;
 	if(primary && engine_val.highlight_ratio != 1.0) {
-		result.highlight_ratio = result.highlight_ratio + engine_val.highlight_ratio - 1.0; // selected unit
+		result.highlight_ratio = result.highlight_ratio * engine_val.highlight_ratio; // selected unit
 	}
 
 	assert(engine_val.offset == 0);