We shouldn't step into a csideimg_ scene

Stepping into a gosub/goto scene will load the scene in order to continue showing the steps appropriately. Actually opening csideimg scenes in the editor is a bad idea for performance reasons, but it's also not a particularly helpful thing to do.
When stepping this can seemingly cause CSIDE to crash (it often doesn't, but loading these scenes takes _a long_ time, so people will think it has).

Stepping should ignore/skip over csideimg_ files, where it can (might be difficult if the name is derived from an expression?).