deathpickups: some improvements

[sacr1ficez]

• Fixed pickup event handler (the code never passed through matchingDimension check because it was always nil).
• get methods are now called just on script init & when certain setting changes in resource.
• Improved readability & optimisation.

[jlillis]

Looks good overall. Use of temporary variables makes this script longer than it needs to be, but perhaps more readable. Changes requested are minor and optional. Ready for merge later this week absent any other blocking comments.