Update to the HL 25th anniversary engine

[SaintWish]

This will allow us to take advantage of the 25th's fog system that uses shaders and help mitigate the GL_MAXTEXTURES crash since it increases the amount to 10k.

[SaintWish]

On TFC trigger_push on some thirdparty maps are broken, so we might have to adjust our maps?

[tschumann]

I don't think trigger_push should be an issue as long as you don't update the entity code. I think the only thing that should be required to get the game running is updating the Steamworks library to v1.53a.

The VGUI stuff might be a bit wonky too from experience but it should at least load.

[SaintWish]

Ah okay cool

[tschumann]

Not sure about the best way to transition - maybe a different branch on Steam with the new engine?

[SaintWish]

Yeah that's probably what I'll do to test.