Fixed iOS NumericOverFlow Exceptions 

for Color components and Constantbuffer PlatformInitialize

I have been getting numericoverflow exceptions in iOS after updating to Xamarin.iOS 7.2 with latest version of Xcode, VS2013 update 2. 

This is a simple bit mask/shift fix to ensure that the values are never larger than Byte which solves the issue. There may be other places that require this but I have only found these so far.