\pgfmathrand & co inaccuracies

[xworld21]

Three things:

• trivial (and urgent?): \pgfmathrand@ should return a number between -1 and 1, but now it's between 1 and 3 (a minus sign is missing!)
• easy: \pgfsetmathseed should be set to call srand in order to make the output deterministic (or rather, the corresponding low level primitive should do that)
  • corollary of the above, SOURCE_DATE_EPOCH should trigger a deterministic call to srand, since PGF derives the default seed from the current time
• hard, but not that much: LaTeXML should use the same random number generator as PGF so that it generates the same image when the seed is specified