OpenBVE v1.7.1.1

This is a maintenence release, aimed at improving the performance of the new renderer introduced in OpenBVE 1.7.1.0.

In most cases, it should now be either considerably faster than the old renderer, or at most a little slower. Work is still ongoing in this regard, and there are hopefully more boosts in the pipeline.

It however introduces one new errata item concerning polygon lighting- https://github.com/leezer3/OpenBVE/wiki/Errata#lighting-behaviour-with-the-same-texture-defined-for-both-daytime-and-nighttime