Merge branch 'PrincipalAxisRendering' into 'master'

Principal axis rendering

- use instancing when hardware support available
- added 3d arrow mode with instancing
- optimize vertex format of 2d line mode (non instanced branch),  see issue #71

See merge request !141

Shaders/PrincipalAxisNode/3d_instanced.glsl

@@ -13,12 +13,13 @@ uniform float pa_cone_radius;
 
 uniform vec3 pa_scale;
 uniform vec3 pa_offset;
+uniform int pa_visible_tensors;
 
 void main()
 {
   SG_VERTEX_BEGIN;
   
-  int pa_component = gl_InstanceID % 3;
+  int pa_component = gl_InstanceID % pa_visible_tensors;
   
   vec4 pa_posOS = SG_INPUT_POSOS;
   

Shaders/PrincipalAxisNode/lines_extruded_instanced.glsl

@@ -16,12 +16,13 @@ uniform vec2 pa_screenSize;
 
 uniform vec3 pa_scale;
 uniform vec3 pa_offset;
+uniform int pa_visible_tensors;
 
 void main()
 {
   SG_VERTEX_BEGIN;
   
-  int pa_component = gl_InstanceID % 3;
+  int pa_component = gl_InstanceID % pa_visible_tensors;
   
   // positions of the line end-points
   vec4 pa_posOS[2];