main: Close memory leak of shader string from load_text_file.

main: Close memory leak of shader string from load_text_file.

Could have just added a call to free() to main, but since we're using
talloc everywhere else, we might as well just use it here too. So pass
a new 'ctx' argument to load_text_file.

This removes a single memory leak from all invocations of the
standalone glsl compiler.