WS30: Adjust based vegetation density

Previously, by observation comparing actual vegetation density
on Medium, WS30 had many more trees than WS20.

This adjusts the base vegetation density down to match WS20.

Note that at High and Very High densities, there is still a disparity
that needs to be addressed.

simgear/scene/tgdb/VPBTechnique.cxx

@@ -1397,8 +1397,8 @@ void VPBTechnique::applyTrees(BufferData& buffer, Locator* masterLocator)
     const double C            = r_E_lon * cos(lat);
     const double one_over_C   = (fabs(C) > 1.0e-4) ? (1.0 / C) : 0.0;
     const double one_over_r_E = 1.0 / r_E_lat;
-    const double delta_lat    = sqrt(100.0 / vegetation_density) / r_E_lat;
-    const double delta_lon    = sqrt(100.0 / vegetation_density) / (r_E_lon * cos(loc.getLatitudeRad()));
+    const double delta_lat    = sqrt(1000.0 / vegetation_density) / r_E_lat;
+    const double delta_lon    = sqrt(1000.0 / vegetation_density) / (r_E_lon * cos(loc.getLatitudeRad()));
 
     const osg::Matrix rotation_vertices_c = osg::Matrix::rotate(
      M_PI / 2 - clat, osg::Vec3d(0.0, 1.0, 0.0),