Add alternate shadow volume rendering pathway (layered geo)

shader/shadow/cap.geom

@@ -1,5 +1,6 @@
 #version 450 core
 layout (triangles) in;
+layout (invocations = 20) in;
 layout (triangle_strip, max_vertices = 3) out;
 layout (std140, binding = 0) uniform TheMat { mat4 theMat; } ;
 layout (std140, binding = 1) uniform LightsBlock
@@ -7,8 +8,7 @@ layout (std140, binding = 1) uniform LightsBlock
     vec4 posBool[20];
     vec4 colRad[20];
 } ;
-uniform int lightID;
-vec3 lightPos = posBool[lightID].xyz ;
+vec3 lightPos = posBool[gl_InvocationID].xyz ;
 float theBool = posBool[gl_InvocationID].w;
 // this code is duplicated in lineShadow.geom, should not be changed on its own
 vec4 projNear (vec4 pos)
@@ -35,8 +35,11 @@ void main()
     {
         // the front cap
         vec4 v1 = vec4 (0,0,1,0) ;
+        gl_Layer = gl_InvocationID;
         gl_Position = theMat * projNear(p0); EmitVertex();
+        gl_Layer = gl_InvocationID;
         gl_Position = theMat * projNear(p1); EmitVertex();
+        gl_Layer = gl_InvocationID;
         gl_Position = theMat * projNear(p2); EmitVertex();
         EndPrimitive();
     }