Fix compilation on non-MSVC

The top two commits fix compilation on non-MSVC.

Specifically:
- Removed extra class qualification
- Renamed _vptr (reserved!)

The latter three resolved some compiler warnings. The one with the virtual destructor is quite necessary, though; ScriptBase offers virtual methods after all.

There's still another variable shadowing warning here; but I didn't know how to best resolve that one:
engines/bladerunner/vqa_decoder.cpp:356:15: warning: declaration of ‘uint32 size’ shadows a parameter [-Wshadow]

There's also a few "variable set but not used" warnings. I ignored them for now since the engine is WIP after all. :)