Don't reset the random texture base when rebuilding a cloud layer

Don't reset the random texture base when rebuilding a cloud layer
This fix removes obnoxious visuals (texture jumping) when a cloud
layer is moved due to a metar update or, more significantly, when
switching from metar to a scenario. Also, I switched to using a TexMat
to displace the cloud texture in order to avoid writing the texture
array every frame.