SLA gizmo clipping plane logic moved to fragment shader

This means the clipping now works again with both legacy and modern OpenGL

src/slic3r/GUI/3DScene.hpp

@@ -427,6 +427,9 @@ private:
     // z range for clipping in shaders
     float z_range[2];
 
+    // plane coeffs for clipping in shaders
+    float clipping_plane[4];
+
 public:
     GLVolumePtrs volumes;
 
@@ -485,6 +488,7 @@ public:
     }
 
     void set_z_range(float min_z, float max_z) { z_range[0] = min_z; z_range[1] = max_z; }
+    void set_clipping_plane(const double* coeffs) { clipping_plane[0] = coeffs[0]; clipping_plane[1] = coeffs[1]; clipping_plane[2] = coeffs[2]; clipping_plane[3] = coeffs[3]; }
 
     // returns true if all the volumes are completely contained in the print volume
     // returns the containment state in the given out_state, if non-null