Fixed MOAIEnvironment.screenDpi nil issue for iOS host

That fix sets MOAIEnvironment.screenDpi parameter in MoaiView maoiInit method for iOS host.
That solution is not ideal - there are two hardcoded dpi values (for iPhone and iPad) and the resulting dpi is being calculated using interface idiom macros and current scale factor (retina/non retina display).
Also it will report lower than real dpi value for iPad Mini, but it seems to be no appropriate workaround yet .