[DVIPDFMx] changing PDF default to 1.5

[Karl Berry]

(Feel free to forward.)

Elie Roux suggested on the tex-live list and comp.text.tex
(http://groups.google.com/group/comp.text.tex/browse_thread/thread/5a0bf450d5f787ab)
to have TeX output PDF 1.5 by default, so we can take advantage of
additional compression.  Meaning these two lines in TeX Live's .ini files:

     \pdfminorversion=5
     \pdfobjcompresslevel=2

Although we could change just (for example) luatex without changing
anything else, it seems more logical to benefit everything possible, if
we're going to make the change: luatex and pdftex for sure, perhaps also
xetex and dvipdfmx if they can also do the compression.

The programs Elie has tested all work fine with 1.5:

    Adobe supports PDF 1.5 from version 6, mupdf, xpdf, poppler, xpdf,
    gnupdf support it too

And pdftex can include a 1.4 pdf in 1.5 output (and vice versa :).

The compression gain varies with the document, of course, but it's not
zero: Elie reports 10-25% on his various test documents.

We could also make it an option at TL install time, but of course it's
better not to introduce options if we can just improve the default.

So that's the question: is it safe and a good idea to have pdftex and
luatex output 1.5 by default, with object compression turned on?

Please advise.

Thanks,
Karl