- added a StartSlideshow ACS and ZScript command and extended the functionality to specify the slideshow's name when starting it.
This is for triggering any kind of intermission definition in the middle of a level - keep in mind that this may not be set up to loop!
This method was chosen because it avoids adding variable declarations to the global namespace which would have required a lot more work while polluting the grammar.
This way the global variables can be handled by a small bit of special coding in the struct generator.
Note that this completely disables the newly added keywords 'play' and 'ui' for unversioned code to allow using them as identifiers as I have found at least one mod that uses a variable named 'play' that would have been rendered broken otherwise.
This also disables many ZScript only keywords for other parsing jobs.
