render/color: Invert ownership model of color_transform types.

Color transform can have multiple types and these different types
want to store different metadata. We previously stored this metadata
directly on wlr_color_transform even for transforms that don't use it.

Instead, let's take the prior art from wlr_scene where each scene node
is built on a base node. Notice how wlr_color_transform_lut3d now has
a `struct wlr_color_transform base`. This is advantageous in multiple
ways:

1. We don't allocate memory for metadata that will never be used.
2. This is more type safe: Compositors can pass around a
struct wlr_color_transform_lut3d if they know they only want to use a
3d_lut.
3. This is more scalable. As we add more transform types, we don't have
to keep growing a monolithic struct.

render/color.c

@@ -15,7 +15,15 @@ struct wlr_color_transform *wlr_color_transform_init_srgb(void) {
 }
 
 static void color_transform_destroy(struct wlr_color_transform *tr) {
-	free(tr->lut3d.lut_3d);
+	switch (tr->type) {
+	case COLOR_TRANSFORM_SRGB:
+		break;
+	case COLOR_TRANSFORM_LUT_3D:;
+		struct wlr_color_transform_lut3d *lut3d =
+			wlr_color_transform_lut3d_from_base(tr);
+		free(lut3d->lut_3d);
+		break;
+	}
 	wlr_addon_set_finish(&tr->addons);
 	free(tr);
 }
@@ -35,3 +43,10 @@ void wlr_color_transform_unref(struct wlr_color_transform *tr) {
 		color_transform_destroy(tr);
 	}
 }
+
+struct wlr_color_transform_lut3d *wlr_color_transform_lut3d_from_base(
+		struct wlr_color_transform *tr) {
+	assert(tr->type == COLOR_TRANSFORM_LUT_3D);
+	struct wlr_color_transform_lut3d *lut3d = wl_container_of(tr, lut3d, base);
+	return lut3d;
+}