#39 Sky cloud rendered poorly

[Mike]

plib 1.8.5
examples 1.8.5

Both WinXP and MacOSX

Steps to reproduce:
Run the Sky example
Move altitude to just above 1500
The middle cloud layer is suddenly rendered poorly: the edges aren't anti-aliased as well.

Note that if you change the cloud layers' altitudes to be in order
#1 at altitude 1000
#2 at altitude 2000
#3 at altitude 3000
then this problem does not appear.