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/node_combobox.h

@@ -0,0 +1,12 @@
+#pragma once
+#include "node_button.h"
+
+class NodeComboBox : public NodeButton
+{
+public:
+    std::array<std::string, 4> labels{ "Normal", "Multiply", "Screen", "Color Dodge" };
+    int m_current_index = 0;
+    virtual Node* clone_instantiate() const override;
+    virtual void clone_copy(Node* dest) const override;
+    virtual void loaded() override;
+};