Fixed: Checking if a CPTextField is usable in a CPPlatformWindow

Previously, when a CPTextField was in a CPPlatformWindow, the checking of if the CPTextField was usable or not (depending on its position) in a CPPlatformWindow didn't take in account the content-inset of the CPTextField. Now it does.
Merge pull request #2171 from Dogild/CPTextField-Platform

Fixed: Checking if a CPTextField is usable in a CPPlatformWindow