internal mptopdf conversion fails on older mps file (using MkIV, beta 2013.12.26)

[Sanjoy Mahajan <sanjoy@olin.edu>]

The MkIV internal mptopdf converter fails on the following minimal example.
Here is the tex:

------- cut test2.tex here -------------
\enabletrackers[graphics.conversion,graphics.inclusion,fonts.mapfiles,fonts.loading,fonts.missing,fonts.mapping]
\starttext
\externalfigure[fig.1][frame=on]
\blank
\externalfigure[fig-1.pdf][frame=on]
\stoptext
------- cut test2.tex here -------------

And here is the figure, which was produced with mpost 0.993 (and also
converted to fig-1.pdf using the 'mptopdf' command):

-------- cut fig.1 here ------------
%!PS-Adobe-3.0 EPSF-3.0
%%BoundingBox: 0 0 10 10 
%%HiResBoundingBox: 0 0 9.96262 9.96262 
%%Creator: MetaPost 0.993
%%CreationDate: 2009.10.21:1318
%%Pages: 1
%%BeginProlog
/bd{bind def}bind def
/hlw{0 dtransform exch truncate exch idtransform pop setlinewidth}bd
/vlw{0 exch dtransform truncate idtransform setlinewidth pop}bd
/l{lineto}bd/r{rlineto}bd/c{curveto}bd/m{moveto}bd/p{closepath}bd/n{newpath}bd
/C{setcmykcolor}bd/G{setgray}bd/R{setrgbcolor}bd/lj{setlinejoin}bd/ml{setmiterlimit}bd
/lc{setlinecap}bd/S{stroke}bd/F{fill}bd/q{gsave}bd/Q{grestore}bd/s{scale}bd/t{concat}bd
/sd{setdash}bd/rd{[] 0 setdash}bd/P{showpage}bd/B{q F Q}bd/W{clip}bd
%%EndProlog
%%Page: 1 1
 0.7 G
n 0 0 m
0 9.96262 l
9.96262 4.98131 l
 p F
P
%%EOF
-------- cut fig.1 here ------------

Running it through MkIV with the 2013.12.26 beta (or the stable context)
produces an empty frame for the inclusion of fig.1, but the correct
triangle for the inclusion of fig-1.pdf.  The empty frame has the
correct size, so the parser is getting at least as far as the bounding
box.

If I regenerate fig.1 using a recent metapost (1.803), the mps file is
simpler, and it is incorporated correctly by context.

-------- cut fig.1 produced with recent metapost here ------------
%!PS
%%BoundingBox: 0 0 10 10 
%%HiResBoundingBox: 0 0 9.96262 9.96262 
%%Creator: MetaPost 1.803
%%CreationDate: 2014.01.02:1250
%%Pages: 1
%%BeginProlog
%%EndProlog
%%Page: 1 1
 0.7 setgray
newpath 0 0 moveto
0 9.96262 lineto
9.96262 4.98131 lineto
 closepath fill
showpage
%%EOF
-------- cut fig.1 produced with recent metapost here ------------

Here is the source file:

--- cut fig.mp here -----
def darkshaded = withgreyscale 0.70 enddef;
beginfig(1)
  fill (origin--(0,1)--(1,1/2)--cycle) scaled 10pt darkshaded;
endfig;
--- cut fig.mp here -----

-Sanjoy
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