glsl: Add utility to convert text files to C strings

glsl: Add utility to convert text files to C strings

Will be used to convert the .glsl source file containing software fp64
routines to a .h file that can be included while building the compiler.

This commit contains two squashed together: the first from Ian adding
the utility (with the existing title), and the second from Dylan making
the code both python2 and python3 compatible.

This is somewhat modeled after the xxd utility that comes with Vim.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
xxd.py: Make python2 and 3 compatible

This makes use of unicode_literals, so that undecorated strings are
considered text (python2 unicode, python3 str) and not bytes in python2
and text in python3. It makes use of io.open, which provides python2
with python3's open behavior (it's an alias in python3), in particular
support for the 't' and 'b' option. Finally, it decorates all of the
string literals with the 'b' prefix, so that python interprets them as
bytes.

I've removed the stdin and stdout options, as python2 always requires
these to be bytes, but python3 always treats them as text (there is a
way to get at the underlying bytes buffer, but that's even more
complexity), and makes the input files required arguments.

In the meson we use the '@INPUT@' shorthand instead of listing each
input, as meson will expand that to [prog_python, '@INPUT0@', @INPUT1@,
..., @OUTPUT@, ...]