Add MinSlope groundcover setting

Here's code to add a minimum slope configuration option to the groundcover feature in T3D 1.2.

I find it useful as a way to keep grass off roads, plus it provides an easy and completely procedural means of diversifying ground cover for variety.

All the changes are in groundCover.cpp and groundCover.h. Just find the matching maxslope and add the minslope portion to your code. It's an easy patch.

One patched and compiled, you will find the configuration option in the Types section.

these are the change to groundCover.cpp:

      mMinSlope[i] = 0.0f;
      mMaxSlope[i] = 0.0f;

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         addField( "minSlope",      TypeF32,       Offset( mMinSlope, GroundCover ), MAX_COVERTYPES,        "The minimum slope angle in degrees for placement." );

         addField( "maxSlope",      TypeF32,       Offset( mMaxSlope, GroundCover ), MAX_COVERTYPES,        "The maximum slope angle in degrees for placement." );

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         stream->write( mMinSlope[i] );
	 stream->write( mMaxSlope[i] );

---

         stream->read( &mMinSlope[i] );
	 stream->read( &mMaxSlope[i] );

---

      const F32 typeMinSlope = mMinSlope[type];
      const F32 typeMaxSlope = mMaxSlope[type];

---

         // Do we need to check min slope?
         if ( !mIsZero( typeMinSlope ) )
         {
            if (mAcos(normal.z) < mDegToRad(typeMinSlope))
               continue;
         }

         // Do we need to check slope?
         if ( !mIsZero( typeMaxSlope ) )
         {
            if (mAcos(normal.z) > mDegToRad(typeMaxSlope))
               continue;
         }
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this is the change to groundCover.h:

   /// The maximum slope angle in degrees for placement.
   F32 mMinSlope[MAX_COVERTYPES];

   /// The maximum slope angle in degrees for placement.
   F32 mMaxSlope[MAX_COVERTYPES];

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