forward declare Point3D in sg_types.hxx

This avoids sucking in SGMath.hxx -- and therefore a bunch of OSG headers --
into many compilation units.

simgear/io/sg_binobj.hxx

@@ -33,6 +33,7 @@
 #include <simgear/compiler.h>
 #include <simgear/constants.h>
 #include <simgear/math/sg_types.hxx>
+#include <simgear/math/point3d.hxx>
 #include <simgear/bucket/newbucket.hxx>
 
 #include <stdio.h>

simgear/math/sg_types.hxx

@@ -38,7 +38,7 @@
 #include <string>
 #include <vector>
 
-#include <simgear/math/point3d.hxx>
+class Point3D;
 
 using std::vector;
 using std::string;

simgear/misc/texcoord.cxx

@@ -149,6 +149,7 @@ enter this in the official comments in case I forget again. :-)
 
 #include "texcoord.hxx"
 
+#include <simgear/math/point3d.hxx>
 // using std::cout;
 // using std::endl;
 

simgear/scene/sky/oursun.cxx

@@ -41,6 +41,7 @@
 #include <osg/Texture2D>
 #include <osgDB/ReadFile>
 
+#include <simgear/math/SGMath.hxx>
 #include <simgear/misc/PathOptions.hxx>
 #include <simgear/screen/colors.hxx>
 #include <simgear/scene/model/model.hxx>

simgear/scene/tgdb/SGLightBin.hxx

@@ -22,6 +22,8 @@
 #ifndef SG_LIGHT_BIN_HXX
 #define SG_LIGHT_BIN_HXX
 
+#include <simgear/math/SGMath.hxx>
+
 class SGLightBin {
 public:
   struct Light {