llvmpipe:fix using 32bit rasterization mistakenly, causing overflows

llvmpipe:fix using 32bit rasterization mistakenly, causing overflows

We use the bounding box (triangle extents) to figure out if 32bit rasterization
could potentially overflow. However, we used the bounding box which already got
rounded up to 0 for negative coords for this, which is incorrect, leading to
overflows and hence bogus rendering in some of our private use.

It might be possible to simplify this somehow (we're now using 3 different
boxes for binning) but I don't quite see how.

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>