add transform mode and tollbar button, implement polygon clipping with uvs interpolation and cube faces projection with near plane clipping, add duplicate points removal template function, implement Spere mesh surface section creation.

src/brush.h

@@ -62,7 +62,6 @@ class BrushMesh
 public:
     GLuint buffers[3]{ 0 };
     GLuint vao{ 0 };
-    struct vertex_t { glm::vec4 pos; glm::vec2 uvs; };
     struct instance_t { glm::mat4 mvp; float flow; };
     int loc_flow = 0;
     int loc_mvp = 0;