From reading the wiki

texsrc : if 0 (default), the triangle's texture is read from SPRITES RAM. If 1, the texture comes from the MAP RAM.
If 2, the texture comes from the screen RAM in the next VBANK (the one following the VBANK on which the ttri() is set to be displayed) (e.g., a ttri() on vbank(0) with the texsrc=2 will use vbank(1) as its texture at the time of its execution). Although there is technically no vbank(2), this can be used on a ttri() with texsrc=2 that's rendered on vbank(1).

Looks like this was added to the lua api in this commit and I just missed it