llvmpipe: Bump the maximum texture size (in pixels).

llvmpipe: Bump the maximum texture size (in pixels).

But cap the size in bytes, to avoid depleting the whole system memory,
with humongus textures.

Tested with max-texture-size piglit test.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>