Replace glBuild2DMipmaps with GL_GENERATE_MIPMAPS

Quote "gluBuild2DMipmaps. Never use this."
gluBuild2DMipmaps was causing massive stutters on many maps on windows
this replaces it with GL_GENERATE_MIPMAP and removes glBuild2DMipmaps completely

https://www.khronos.org/opengl/wiki/Common_Mistakes