Merge animation subfiles when importing material presets

[snailspeed3]

In Mario Kart Wii, track .brres files only support 8 animation subfiles of a particular type (e.g. SRT0), which is problematic, as mass material preset import creates (in the worst case) multiple individual animation subfiles per material imported--sometimes above 40. In this circumstance, the material preset mass import-export workflow is untenable.