glsl: Move functions into standalone_scaffolding.cpp for later reuse.

glsl: Move functions into standalone_scaffolding.cpp for later reuse.

This patch moves the following functions from main.cpp (the main cpp
file for the standalone executable that is used to create the built-in
functions) to standalone_scaffolding.cpp, so that they can be re-used
in other standalone executables:

- initialize_context()*
- _mesa_new_shader()
- _mesa_reference_shader()

*initialize_context contained some code that was specific to main.cpp,
so it was split into two functions: initialize_context() (which
remains in main.cpp), and initialize_context_from_defaults() (which is
in standalone_scaffolding.cpp).