llvmpipe: use triangle subdivision to avoid fixed-point overflow issues

llvmpipe: use triangle subdivision to avoid fixed-point overflow issues

If we're drawing to a surface that's 2048 x 2048 pixels or larger there's
danger of fixed-point overflow in the triangle rasterization code.  That
leads to various rendering glitches.

Rather than implement some intricate changes to the rasterization code,
simply subdivide triangles into smaller subtriangles to avoid the issue.
Only do this when the drawing surface is larger than 2048 by 2048.

Reviewed-by: José Fonseca <jfonseca@vmware.com>