svga: implement support for signed byte vertex attributes

There's no SVGA3D_DECLTYPE that directly corresponds to
PIPE_FORMAT_R8G8B8_SNORM.  Previously, we used the swtnl fallback
path to handle this but that's slow and causes invariance issues.
Now we fetch the attribute as SVGA3D_DECLTYPE_UBYTE4N and insert
some extra VS instructions to remap the attributes from the range
[0,1] to the range[-1,1].

Fixes Sauerbraten sw fallback.
Fixes piglit normal3b3s-invariance test.

Reviewed-by: Charmaine Lee <[email protected]>

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