fix conflicting bbox setting between pgfsys-dvisvgm and dvisvgm command;

[agrahn]

Motivation for this change

fixes #1275

Checklist

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• [x] Code changes are licensed under GPLv2 + LPPLv1.3c
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[agrahn]

@hmenke, @muzimuzhi Do you think this PR could be merged? If something is still missing, please let me know.