gfx_starfield: new Starfield Simulation inspired module

[cyriax0]

I added an effect inspired by the old Starfield Simulation screensaver for windows.

Working

• flying through the starfield
• stars are generated as they leave the field
• star density (defaults to 1star/100px)

Possible Perfomance Issues

I use matrix_clear() each frame, IDK if that could be improved.

Possible Eyecandy Improvements

• better colored stars
• different flight modes
  • changing speed
  • rotation
  • warping

[vifino]

A simple tweak would be to use HSV for the colors, store the hue per star and randomize that one. You probably thought of that too. ;)

Address the changes and I'll gladly merge it.