Merge 8fcf59f883 into acb92944d4

Currently monitors which are mirroring another one are returned by
`glfwGetMonitors` but their `ns.screen` is `NULL`. Some functions like
`glfwGetMonitorContentScale` and `glfwGetMonitorWorkarea` assert that
`ns.screen` is not `NULL`. Since mirrored monitors are logically the
same we should only consider the primary monitor and ignore the
non-primary ones.

src/cocoa_monitor.m

@@ -319,6 +319,9 @@ void _glfwPollMonitorsCocoa(void)
         if (CGDisplayIsAsleep(displays[i]))
             continue;
 
+        if (CGDisplayMirrorsDisplay(displays[i]) != kCGNullDirectDisplay)
+           continue;
+
         const uint32_t unitNumber = CGDisplayUnitNumber(displays[i]);
         NSScreen* screen = nil;