Bug 789728 - Screenshot whole screen (Windows 10) only grabs screen...

... from first monitor

While researching the cause for the missing window contents (bug
793722), I noticed that the full screen capture mode was also not
working as expected. No matter how many monitors were connected, it only
ever captured the contents of the main monitor. This patch adjusts the
source rectangle for the BitBlt copy operation so that multiple monitors
are captured correctly.