add ComboBox node, add blend modes button in stroke panel, move brush shader code to ShaderManager and avoid the same shader being recompiled many times

engine/brush.h

@@ -39,7 +39,6 @@ struct StrokeSample
 class BrushMesh
 {
 public:
-    ui::Shader shader;
     GLuint buffers[3]{ 0 };
     GLuint vao{ 0 };
     struct vertex_t { glm::vec4 pos; glm::vec2 uvs; };