Add support for --stdout

With the recent --stdin option, it would perhaps also be useful to have support for --stdout; so the use of the filesystem can be completely avoided when using `glslangValidator` in a programatic way (e.g. from a build system or via subprocess).

(Additionally, glslang should print errors to stderr, not stdout)

Start with glslangValidator's existing command-line handling for --stdin and trace how input and output streams are selected. Done means --stdout avoids filesystem use for programmatic invocation and errors are emitted on stderr rather than stdout.

Written by the indexing model from the issue text.