Clear the VirtualDevice we create for an impBufferDevice on iOS

Helps rendering of complex SmartArt with shapes a lot, especially such
that include shadows. Exact mechanism unknown. Possibly on other
platforms virtual devices get implicitly "cleared" when created? Sigh,
fun.

SmartArt is still not rendered fully like on desktop (OS X), though.

Change-Id: I1bf92e85b5659ab7f9f4d843b77619de3af36d10