Import some 2D graphics and shader files from Turtle Arena

There have been some changes from Turtle Arena for q3 compatiblity, but still more to be done.
Import common textures from Turtle Arena
Add background placeholder
Import some missing UI gaphics

From Turtle Arena's base_ioq3ztm.
Remove Turtle Arena randombot icon source
Import skirmish menu button

From Turtle Arena's base_ioq3ztm
Remade addbotframe and cut_frame as SVG
Remade frame1_l, frame1_r, and frame2_l as 4x SVG