Quick fixes & optimizations for custom battles on the experimental branch.

Additions

• Added graphics options for terrain quality with 'Track trail effects' option enabled. Disabling track trails should get you back most of any performance lost in V0.124.

Fixes

• Fixed screen flashing black seemingly randomly due to v0.124 changes.
• Several stability improvements for custom battles.

Optimizations

• Sped up rendering of 'Dunes' map terrain.