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/shader.h

@@ -41,6 +41,7 @@ enum class kShader : uint16_t
     StrokePreview = const_hash("stroke-preview"),
     Checkerboard= const_hash("checkerboard"),
     Equirect    = const_hash("equirect"),
+    BrushStroke = const_hash("brush-stroke"),
 };
 
 class Shader
@@ -67,6 +68,7 @@ public:
     static bool create(kShader id, const char* vertex, const char* fragment);
     static void use(kShader id);
     static void use(const char* name);
+    static Shader* get(kShader id);
     static void u_vec4(kShaderUniform id, const glm::vec4& v);
     static void u_vec2(kShaderUniform id, const glm::vec2& v);
     static void u_mat4(kShaderUniform id, const glm::mat4& m);