Added missing texture that was causing issues in the Emscripten build.

Removed some old unneeded texture loading classes.
Added complete nullTexturemanager implementation.
Added source and library for SOIL2 (not yet used)
Updated texture loading classes to use the renderer's texture manager as opposed to the Texture class.
Commented out all old Texture code that didn't involve holding data.
Added updated edition of stb_image.h.
Removed old stb_image.c.
Added STBTextureManager class to handle texture loading using stb_image.