llvmpipe: Ensure the 16x16 special rasterization path does not touch outside the...

llvmpipe: Ensure the 16x16 special rasterization path does not touch outside the tile.

llvmpipe has a few special rasterization paths for triangles contained in
16x16 blocks, but it allows the 16x16 block to be aligned only to a 4x4
grid.

Some 16x16 blocks could actually intersect the tile
if the triangle is 16 pixels in one dimension but 4 in the other, causing
a buffer overflow.

The fix consists of budging the 16x16 blocks back inside the tile.