ui/app: (macOS) use a consistent and constant backing store constant

Compute and keep constant the pixel density, to avoid window content
changes when moving to another monitor with a different density.

Use backing store scale, not content scale for input events.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/gl_macos.m

@@ -13,7 +13,7 @@
 
 static void handleMouse(NSView *view, NSEvent *event, int typ, CGFloat dx, CGFloat dy) {
 	NSPoint p = [view convertPoint:[event locationInWindow] fromView:nil];
-	CGFloat scale = view.layer.contentsScale;
+	CGFloat scale = view.window.backingScaleFactor;
 	if (!event.hasPreciseScrollingDeltas) {
 		// dx and dy are in rows and columns.
 		dx *= 10;