UI: Center preview spacing lines

Currently, the spacing lines begin in the middle of the box and then are
their width wide towards one direction. This means that the larger they
are, the more off-center their middle is.
This commit changes them to start at half their width left/top of the
center and end half the width to the right/bottom of the center, putting
the middle in the center.

UI/window-basic-preview.cpp

@@ -1758,13 +1758,16 @@ static void DrawLine(float x1, float y1, float x2, float y2, float thickness,
 
 	gs_render_start(true);
 
-	gs_vertex2f(x1, y1);
-	gs_vertex2f(x1 + (xSide * (thickness / scale.x)),
-		    y1 + (ySide * (thickness / scale.y)));
-	gs_vertex2f(x2 + (xSide * (thickness / scale.x)),
-		    y2 + (ySide * (thickness / scale.y)));
-	gs_vertex2f(x2, y2);
-	gs_vertex2f(x1, y1);
+	gs_vertex2f(x1 - (xSide * (thickness / scale.x) / 2),
+		    y1 + (ySide * (thickness / scale.y) / 2));
+	gs_vertex2f(x1 + (xSide * (thickness / scale.x) / 2),
+		    y1 - (ySide * (thickness / scale.y) / 2));
+	gs_vertex2f(x2 + (xSide * (thickness / scale.x) / 2),
+		    y2 + (ySide * (thickness / scale.y) / 2));
+	gs_vertex2f(x2 - (xSide * (thickness / scale.x) / 2),
+		    y2 - (ySide * (thickness / scale.y) / 2));
+	gs_vertex2f(x1 - (xSide * (thickness / scale.x) / 2),
+		    y1 + (ySide * (thickness / scale.y) / 2));
 
 	gs_vertbuffer_t *line = gs_render_save();