Errorneous width calculation in point renderer

[laihro]

Hello dear K3D team,

the point render and possibly also the other renderers errorneously assume larger world coordinate Z values to appear closer to the user. The center of the "world" is [0,0,0]. More distant Z-Values in fact become larger while the closer Z-Values becomer smaller e.g. [0,0,50] is more distant to the user than [0,0,-50]. Thus, the point width calculation

  c = worldcoords[i].z + dscale;

is wrong and needs to be changed to

  c = (-1*worldcoords[i].z) + dscale;

See the attached patch.