keep screen from dimming down when running certain app?
That's easily possible using Tasker (I do the same for selected reading apps to increase the timeout to 2min, while also having the 30s default for all other apps):


Condition: App (select the apps you want to keep the screen active for)
Task: Display Timeout (configure the timeout interval using the sliders)


Alternatively, in "Task" you could also let Tasker switching the keyguard off to disable the lock screen.

At least concerning the display timeout adjustment, this might be possible with other automation apps as well, such as Llama -- I'm not sure, as I use Tasker I don't use the others. Though Tasker might look a bit pricy (~5 EUR) and complicated, after you figured out how to use it you will find it worth every cent. Until then, there's a free trial available on their homepage.