[metapost] Metafun doesn' fill arrow heads anymore

[Akira Kakuto]

> This bug seems to be related to the fillup macro of Metafun.
> It seems it doesn’t work as it should, at least outside of ConTeXt.

In the most basic command-line case:

%
% mpost --mem=metafun test.mp
%
beginfig(1);
drawarrow (0,0)--(10,10);
endfig;
end.

a correct eps is created.
Thus the problem may be in the luamplib package.

Best,
Akira