Removed the 'DefaultShaders' source file, added as data files.

Note that there's currently no CMake build setup to automatically copy
VectorStorm shaders (or other data files) into a game's data.  There
probably never will be.  Instead, these data files are provided for use
by games which don't want to implement their own default shaders.

Regardless, any VectorStorm game will require a default_f and default_v
shader, for use when rendering anything which doesn't explicitly
reference a custom shader in its material.