Improve scrollbar behavior (#351)

* Change scrollbar behavior

* Leave space at the end of the scrollbar

* Change mid to center

* Use shorter array initialization

* Add space around scrollbar

* Fix scrollbar spacing

* Smooth end of scrollbars

* Add page up and down

* Page up and down on click in scrollbar track

* Add shift pageup to translate horizontally

editor/src/input/mouse.rs

@@ -22,6 +22,10 @@ impl ViewportBounds {
 	pub fn size(&self) -> DVec2 {
 		self.bottom_right - self.top_left
 	}
+
+	pub fn center(&self) -> DVec2 {
+		self.bottom_right.lerp(self.top_left, 0.5)
+	}
 }
 
 #[derive(Debug, Copy, Clone, Default, Eq, PartialEq, Hash)]