Window relative mouse locations provided via NSWindow and NSEvent are
based at 0,1 while screen relative locations use 0,0.  Incorrect
handling of this had crept into other coordinate transformations.  Note
that most of these errors canceled each other out, so the reported
positions of windows, monitors and work areas are unaffected.  This
corrects the cursor position for glfwGetCursorPos and glfwSetCursorPos.

Fixes #1461.