Add content scale queries

This adds glfwGetWindowContentScale and glfwGetMonitorContentScale for
querying the recommended drawing scale factor for DPI-aware rendering.

Parts of this patch are based on code by @ferreiradaselva.

Fixes #235.
Fixes #439.
Fixes #677.
Fixes #845.
Fixes #898.

src/cocoa_platform.h

@@ -135,6 +135,7 @@ typedef struct _GLFWmonitorNS
     CGDirectDisplayID   displayID;
     CGDisplayModeRef    previousMode;
     uint32_t            unitNumber;
+    id                  screen;
 
 } _GLFWmonitorNS;