DefaultShader: make SHADER_SRC a class-wide constant

src/default_shader.d

@@ -8,24 +8,25 @@ class DefaultShader
     public GLint color_idx;
     protected Shader m_program;
 
+    immutable string SHADER_SRC = `
+vertex:
+    uniform mat4 view;
+    in vec2 position;
+    void main(void)
+    {
+        gl_Position = view * vec4(position, 0.0, 1.0);
+    }
+fragment:
+    uniform vec3 color;
+    void main(void)
+    {
+        gl_FragColor = vec4(color, 1.0);
+    }
+`;
+
     this()
     {
-        immutable string shader_src = `
+        m_program = new Shader("program", SHADER_SRC);
-    vertex:
-        uniform mat4 view;
-        in vec2 position;
-        void main(void)
-        {
-            gl_Position = view * vec4(position, 0.0, 1.0);
-        }
-    fragment:
-        uniform vec3 color;
-        void main(void)
-        {
-            gl_FragColor = vec4(color, 1.0);
-        }
-            `;
-        m_program = new Shader("program", shader_src);
         view_idx = m_program.get_uniform_location("view");
         position_idx = m_program.get_attrib_location("position");
         color_idx = m_program.get_uniform_location("color");