To disable the 3dfx splash screen on Windows, you need to set 
the environment variable FX_GLIDE_NO_SPLASH to 1.

Within a game, this can be done as follows:

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_putenv("FX_GLIDE_NO_SPLASH=1");

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before loading the OpenGL driver, assuming you are using dynamic 
loading (see the Windows Support section).

If you are not using dynamic loading, you can set this variable from
a batch file before launching the game.

If you are using the Mesa Win32 3dfx driver for windowed rendering on
a Voodoo 1, 2 or Rush, you may see the 3dfx logo in the lower right
hand corner even after you have set the environment variable as 
described above.  This can apparently be dealt with by setting 
the environment variable FX_GLIDE_SHAMELESS_PLUG to be empty, i.e.

"SET FX_GLIDE_SHAMELESS_PLUG=" from a DOS prompt / batch file, or

_putenv("FX_GLIDE_SHAMELESS_PLUG=") before dynamically loading the driver.