Add "cap" shadow shader

shader/lighting/cap.geom

@@ -0,0 +1,19 @@
+#version 430 core
+layout (triangles) in;
+layout (triangle_strip, max_vertices = 3) out;
+layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
+uniform vec3 lightPos;
+void main()
+{
+    vec4 p0 = gl_in[0].gl_Position ;
+    vec4 p1 = gl_in[1].gl_Position ;
+    vec4 p2 = gl_in[2].gl_Position ;
+    if ( dot( lightPos, cross( p0.xyz - p1.xyz, p1.xyz - p2.xyz)) < 0)
+    {
+        gl_Position = theMat * p0; EmitVertex();
+        gl_Position = theMat * p1; EmitVertex();
+        gl_Position = theMat * p2; EmitVertex();
+        EndPrimitive();
+    }
+    else {}
+}