Improve performance of battle menus

[fatcerberus]

The frame rate tends to chug while battle menus are open due to rendering text every frame. This could be sped up significantly by pre-rendering menu items before the menu is displayed.

[fatcerberus]

It turns out the text is only part of the problem: all the battle UI is slow in general due to two factors: 1. heavy use of “immediate mode” rendering, e.g. Prim and Sphere v1 primitives, which sends vertices to the GPU endlessly; and 2. tons of temporary Color objects being created and discarded every frame, causing heavy GC pressure.

All of the UI code should be rewritten to use modern rendering techiques, which is not a trivial task.