astro/calcmysky: the port had been updated to version 0.3.1

This release fixes some rendering issues:

• On GPUs that compute mipmaps of non-power-of-two textures poorly eclipsed atmosphere rendering now works as expected
• Atmosphere model examples now don't include Rayleigh phase function in the texture -- this change fixes a "round cap around zenith" artifact

Reported by: portscout