[tlbuild] Compatibility with ConTeXt was broken in asy 2.33

[Mojca Miklavec]

Hi,

There must have been some trivial change in the source code that broke
Asymptote 2.33 in combination with ConTeXt.

If I run
    asy -vvv -render=0 -tex=context filename.asy

I get the following in filename_.tex:

\page\hbox{%
\special{ps:
/ASYx {( ) print ASYX sub 12 string cvs print} bind def
/ASYy {( ) print ASYY sub 12 string cvs print} bind def

And as a consequence images still work, but labels don't.

If I would have to blindly blame one of the latest commits, this could
be one candidate:
    http://sourceforge.net/p/asymptote/code/5691/

Asymptote 2.31 works ok.

Given that this is still a "pretest" phase meant for catching these
kind of problems, I hope that there is still time to include a fix.

(Switching to an older asy is now additionally complicated by the fact
that one also has to switch back to an older ghostscript.)

Thank you very much,
    Mojca