Improve compatibility with different formats of .obj file

When creating custom mountain models, it is not uncommon for someone to come to #modding_help with an unusual exception stemming from somewhere in https://github.com/EverestAPI/Everest/blob/dev/Celeste.Mod.mm/Patches/ObjModel.cs#L26. Typically, the solution is to use blender to re-export their .obj file. The vanilla Celeste code that ObjModel.CreateFromStream() is adapted from was only made with blender's .obj file format in mind and using other tools is very hit or miss.

Support for all styles of .obj files could be more trouble than it's worth, but a few simple things could be done to improve compatibility. Namely, dealing with extra spaces in the file and also potentially adding support for an alternative format I've seen used for the lines starting with `f`. This is something I was meaning to do myself at some point, but I never got around to it, and I figure leaving an issue up might help it get done.