I believe A6.31 requires a video card that supports T&L to run in A6 mode. However, as far as I know A6.31 still has a A5 compatible mode which the engine should use automatically for non-T&L cards (like the Riva TNT2).

See:

d3d_transform
Selects whether vertex transformation and lighting shall be done by the engine A5 , or by DirectX A6 . At start, this variable indicates whether the 3D hardware is T&L capable or not.

Range:
on - T&L by DirectX ( A6 , default on T&L hardware).
off - T&L by engine ( A5 , default on non-T&L hardware).

var d3d_transform = on; // always A6 mode
var d3d_transform = off; // always A5 mode