Use MiniGL as your renderer. Unfortunately, the file r_minigl.dll, which is required to run this, is not included. It, however, can be found here. You must save this into your SRB2 folder, and you will subsequently be able to play in MiniGL.
// tell r_opengl.cpp to compile for MiniGL Drivers
#define MINI_GL_COMPATIBILITY
// tell r_opengl.cpp to compile for ATI Rage Pro OpenGL driver
//#define ATI_RAGE_PRO_COMPATIBILITY
#define DRIVER_STRING "HWRAPI Init(): SRB2 MiniGL renderer"
// Include this at end
#include "../r_opengl/r_opengl.c"
#include "../r_opengl/ogl_win.c"
// That's all ;-)
// Just, be sure to do the right changes in r_opengl.cpp