specify vertex attribute position for rect shader "color" attribute

src/RectShader.cc

@@ -5,7 +5,8 @@ RectShader::RectShader()
     m_program = glcxx::Program::create(
             glcxx::Shader::create_from_file(GL_VERTEX_SHADER, "rect.v.glsl"),
             glcxx::Shader::create_from_file(GL_FRAGMENT_SHADER, "rect.f.glsl"),
-            "coords", 0);
+            "coords", 0,
+            "color", 1);
 
     m_uniforms.viewport_size = m_program->get_uniform_location("viewport_size");
     m_uniforms.position = m_program->get_uniform_location("position");