These two patches allow use of external shaders as the tree and 3d cloud
shaders. The shaders are Shaders/3dcloud.{frag|vert} and
Shaders/tree.{frag|vert}. If the shader files are not found, the
original shaders included in the source are used.
This makes testing modifications to the shaders easier and faster. End
users should see no difference when using this.
Incorporated changes to effects file syntax. Changed Till's landmass
to mix in the base terrain texture. Some shader cleanup.
Also, add a GUI control for enabling shaders.
