OpenGL fixes

Co-authored-by: TehRealSalt <tehrealsalt@gmail.com>

Please, for the love of God, Jesus and Bastet, ADD EVERY OPENGL COMMAND REGARDLESS OF THE FREAKING RENDERER!!!!!!!!!!!!!!!!!!!!!!!!!!!

Don't flush every texture just because the palette MAY have changed

Merge branch 'opengl-is-a-bad-renderer' of https://git.magicalgirl.moe/STJr/SRB2 into opengl-is-a-bad-renderer

(done by calling HWR_LoadTextures, which in turn calls HWR_FreeTextureCache)

Actually don't even call HWR_FreeMipmapCache between levels because I feel like it's futile outside of PWAD loading