Frederic Bouvier:

Frederic Bouvier:

This is a new patch that allow to define a different
texture for top and bottom of clouds. It uses the
overcast_top.rgb you made for me last time.

What the patch do is to install a ssgStateSelector
instead of a ssgSimpleState for each layer.
The SGCloudLayer::draw method is modified to accept
a boolean that will select the proper state: 0/false
for bottom, 1/true for top.

Then, in SGSky::drawUpperClouds and SGSky::drawLowerClouds,
SGCloudLayer::draw is called with false and true because
we see the bottom of upper clouds and the top of
lower clouds.

Only overcast has 2 textures, the other types share the
same state for top and bottom, but that could be modified
in SGCloudLayer::rebuild.