add FIXME about mapping shaders to meshes

src/shader.h

@@ -85,6 +85,10 @@ struct GLmesh
 	GLenum draw_mode; // NOTE: GL_LINES, GL_TRIANGLES
 	GLenum usage; // NOTE: GL_STATIC_DRAW, GL_DYNAMIC_DRAW
 
+	// FIXME: should probably map meshes/entities to shaders in a more abstract
+	// 	way. libTangerine maps each shader with a group of entities in a
+	// 	'render_group' object, but that limits the use of groups to a single
+	// 	shader program
 	shader_program* shader;
 	glm::mat4* model_xform;