llvmpipe: support 8bit subpixel precision

llvmpipe: support 8bit subpixel precision

8 bit precision is required by d3d10 but unfortunately
requires 64 bit rasterizer. This commit implements
64 bit rasterization with full support for 8bit subpixel
precision. It's a combination of all individual commits
from the llvmpipe-rast-64 branch.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>