0b8b6ac56a
It fixes the following issues (to a greater or lesser extent): 1) Performance. Quad trees used to improve culling, and the sprites are placed on the surface of a sphere rather than randomly throughout the cloud, requiring fewer textures. This saves about 5-10fps on my machine. 2) Disabled 3D clouds have no performance impact. Previously they were still in the scenegraph. Now they are removed. 3) Clouds are now loaded on start-up, and don't require the scenario to be changed, they also work with METAR. 4) The cloud field is shifted as you travel. There's a small bug in that the clouds "jump" as you reach the edge of the field. 5) Iterative sorting of sprites. This doesn't appear to solve the alpha blending problem completely, but may help a bit.
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