Multi-monitor fix (linux platform specific)

In the process of working with the “starting point tutorial code” examples I tripped over an assert failure in Desktop::refreshMonitorSizes().

My system has two monitors run by separate Xes, and when clipToWorkArea was false the call to juce_updateMultiMonitorInfo() would add only a single set of monitorCoords. A simple loop on ScreenCount(display) (as in the previous block) solved the problem for me, but probably isn’t a proper fix…?

Ok, I’ve checked in a (hopeful) fix now…