llvmpipe: bump to GL support to GL 4.2

So GL 4.1 has the increased texture limits, this add a check from !5727 (merged) but then bumps the llvmpipe limits.

@sroland @jrfonseca I fully expect this might cause you some issues with embedded and what not, happy to figure out any preferred solution, I had some patches to make the scene allocation be more dynamically allocated somewhere.