Fix invalid binary orbits, hyperjump streaks falling down, negative commodity demand

[Web-eWorks]

Small collection of bugfixes for the upcoming release.

• Fixes #5764 - I broke this by trying to do the gravpoint mass summation pass piece-wise.
• Fixes #5770 - Since stars are rendered in Z-up space, need to compute the direction of hyperspace star "streaks" in Z-up space as well.
• Fixes #5784 - The commodity market already prevented you from selling more items than there was station demand; added a simple filter to prevent other code from putting a commodity market in an invalid state of negative supply/demand.

[impaktor]

@pcercuei Heads up: we're doing a bugfix release in a few hours.