[tex-k] D296: reconstruct pictures with "fill"

[胡亚捷 (Hu Yajie)]

The macros on page 296 of The METAFONTbook reconstruct the picture using
plain METAFONT's "fill" command, which has two obscure problems:

(1) If the user has "screenstrokes" turned on, the macros will stop working
because the first invocation of "fill" will try to open a window, but we
have redefined the primitive token "at" so it will fail with error messages.

(2) "fill" yscales by aspect_ratio, so it may not reconstruct the exact
picture we want, especially if the picture is already yscaled by aspect_ratio.

Using the primitive "addto currentpicture contour" solves both problems.

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Previous reports:
https://github.com/dine2014/texbook-mfbook-bugs