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The previous entry only disabled acceleration on swrast, but softpipe
and llvmpipe shouldn't be used for "GPU" acceleration either.
This should apply to Linux but not ChromeOS, AFAICT.

This only improves an existing software rendering list entry, but here
is the rationale: We prefer to rely on our own (domain specific, so more
efficient) software paths, at least for everything other than WebGL. And
for WebGL, SwiftShader avoids unknown factors like
llvmpipe/softpipe/swrast.

If you are running a Mesa GL driver (not e.g. NVIDIA) then you can force
these configurations with:
- LIBGL_ALWAYS_SOFTWARE=1
  https://docs.mesa3d.org/envvars.html#libgl-environment-variables:~:text=LIBGL_ALWAYS_SOFTWARE
- GALLIUM_DRIVER=llvmpipe, softpipe, or swr (though swr didn't work for me)
  https://docs.mesa3d.org/envvars.html#gallium-environment-variables:~:text=GALLIUM_DRIVER

The GL_RENDERER strings are:
- swrast: "Software Rasterizer" (couldn't test this locally; found this online)
- softpipe: "softpipe" (on one machine)
- llvmpipe: "llvmpipe (LLVM 10.0.0, 256 bits)" (on one machine)

Drive-by updates the description of another item to be more accurate
(SVGA3D is virtualized over hardware; it's not a software renderer).

Bug: 1155974
Change-Id: I0571c1a1bf526260f7ea6cd53f88eec768973b13
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2645491
Commit-Queue: Kai Ninomiya <kainino@chromium.org>
Reviewed-by: Zhenyao Mo <zmo@chromium.org>
Auto-Submit: Kai Ninomiya <kainino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846422}