rewrite of sky dome code

Add more points to the dome, giving it a dome shape rather than a
dunce cap shape.

Represent as OpenGL DrawElements instead of as triangle strips.

Only calculate have the sky colors and reflect those across the dome.

simgear/math/Makefile.am

@@ -20,6 +20,7 @@ include_HEADERS = \
 	sg_random.h \
 	sg_types.hxx \
 	vector.hxx \
+	Math.hxx \
 	SGBox.hxx \
 	SGCMath.hxx \
 	SGGeoc.hxx \

simgear/math/Math.hxx

@@ -0,0 +1,23 @@
+#ifndef SIMGEAR_MATH_MATH_HXX
+#define SIMGEAR_MATH_MATH_HXX 1
+namespace simgear
+{
+namespace math
+{
+/** Linear interpolation between two values.
+ */
+template<typename T>
+inline T lerp(const T& x, const T& y, double alpha)
+{
+    return x * (1.0 - alpha) + y * alpha;
+}
+
+template<typename T>
+inline T lerp(const T& x, const T& y, float alpha)
+{
+    return x * (1.0f - alpha) + y * alpha;
+}
+
+}
+}
+#endif