WScrollBar: More intuitive drag scroll threshold (#91)

Use the movable area (ie. scroll bar length minus
handle length) instead of just the scroll bar length.
This makes it so the handle actually feels like it's sticking
to the cursor while scrolling.

Author: Sturmlilie

src/main/java/io/github/cottonmc/cotton/gui/widget/WScrollBar.java

@@ -130,8 +130,7 @@ public int getMovableDistance() {
 	}
 
 	public int pixelsToValues(int pixels) {
-		int bar = (axis==Axis.HORIZONTAL) ? width-2 : height-2;
-		//int bar = getMovableDistance();
+		int bar = getMovableDistance();
 		float percent = pixels / (float)bar;
 		return (int)(percent*(maxValue-window));
 	}