Some works on the final stage and effects — HellEscape Steam update #814151

- Made Rot Skeletons stronger and their gas a different hue so it won't be confusing. - Added evil trees to Plains of Terror. - Made Evil Eyes a stronger and with bigger range. - Added Pyro Skeleton Mages to plains of terror. - Added enemy: 'Specter'. Watch out, this enemy reduce max health until the end of the mission! - Added enemy: 'Ghoul'. - Improved rain effect and added snow to Snowy Peaks. - Made final boss stronger. - Slightly improved lighting efficiency + added ambient light to some stages.