[osx] Use convertRectFromScreen: instead of convertPointFromScreen
[osx] Use convertRectFromScreen: instead of convertPointFromScreen

As convertPointFromScreen: was added on macOS 10.12 (or 10.14 in the
SDK), we would prefer to use convertRectFromScreen to support macOS
10.9.

Fixes aseprite/aseprite#2838

aseprite/laf,aseprite/laf