MacOS: Fix Cairo rendering on some systems

CGContextConvertSizeToUserSpace returns a negative height on some systems and a
positive height on others.  I have no idea why, but only basic scaling is
required here, so just avoid this function entirely.

Author: drobilla

src/mac_cairo.m

@@ -93,10 +93,8 @@ - (void)drawRect:(NSRect)rect
   const CGSize sizePx = {(CGFloat)view->lastConfigure.width,
                          (CGFloat)view->lastConfigure.height};
 
-  const CGSize sizePt = CGContextConvertSizeToUserSpace(context, sizePx);
-
   // Convert coordinates to standard Cairo space
-  CGContextTranslateCTM(context, 0.0, -sizePt.height);
+  CGContextTranslateCTM(context, 0.0, sizePx.height * scale);
   CGContextScaleCTM(context, scale, -scale);
 
   drawView->surface = cairo_quartz_surface_create_for_cg_context(