Fix Pterabyte spawns (+ behavior tweaks)

After some dev discussion, we concluded that !1380 (merged) was not the right way to nerf Pterabytes. This branch reverts the "mashing" requirement to what it was before, and instead increases the grace period between pickup & getting dropped, from 10 to 70 tics (0.29 to 2 seconds). This should prevent Pterabytes unfairly dropping the player into danger before they can react, while still providing a proper challenge to break free.

While testing these changes, I noticed that Pterabyte spawners broke (never spawning more than 1 Pterabyte) as a result of UDMF-related changes; this branch also fixes that.

srb2win_pterabyte.exe