pdImage: start migrating public variables to scripting dictionary

Each pdImage class now exposes a public scripting dictionary called
imgStorage.  My goal is to eventually phase out non-performance-critical
pdImage public variables in favor of dictionary entries inside
imgStorage.  There are a lot of benefits to this (it's cleaner, much
easier to copy/export large collections of values, much easier to add
new values in the future), but I'm going switch variables over slowly,
so I can make sure there are no unexpected interactions.