The ATI Rage Pro, sadly, utilizes the Mach64 driver.
Mach64 didn't receive a lot of attention to become a very stable or reliable driver for DRI rendering. At best, you can get some 2D acceleration, but otherwise it's not the most stable driver and very else can be done. It's known to crash also if the DRM is loaded, so you may wish to pass the option "NoAccel" "true"
flag in the Section "Device"
section of your xorg.conf file so that the card doesn't crash.
Arch's Wiki has a link to the external kernel module, as by default, no module exists for this video card within the kernel, period. I have no idea how well it performs, but here's a sourcelink:
You're more or less on your own with this one.
You'll also have to download, rebuild, and re-install xf86-video-mach64 with configure option --enable-dri
by hand, possibly (as I honestly can't read the Slackware script to build and install this single stand-alone driver).
I won't post the exact method I use, due to the fact the method I use is different from Slackware's, so you may need to ask Patrick, Robby, or Eric about this...
As far as Mesa3D... it should use the older Tungsten Graphics API driver, but you may want to check and see if it is in the Mesa3D list for compiled DRI drivers... such as this flag for libmesa:
I will warn you, this is possibly entirely unsupported by Slackware officially, and results may vary for you from good to bad.
And for that... I wash my hands of this topic.