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.

include/render/color.h

@@ -4,6 +4,18 @@
 #include <stdint.h>
 #include <wlr/util/addon.h>
 
+enum wlr_color_transform_type {
+	COLOR_TRANSFORM_SRGB,
+	COLOR_TRANSFORM_LUT_3D,
+};
+
+struct wlr_color_transform {
+	int ref_count;
+	struct wlr_addon_set addons; // per-renderer helper state
+
+	enum wlr_color_transform_type type;
+};
+
 /**
  * The formula is approximated via a 3D look-up table. A 3D LUT is a
  * three-dimensional array where each element is an RGB triplet. The flat lut_3d
@@ -19,21 +31,17 @@
  *     offset = 3 * (r_index + dim_len * g_index + dim_len * dim_len * b_index)
  */
 struct wlr_color_transform_lut3d {
+	struct wlr_color_transform base;
+
 	float *lut_3d;
 	size_t dim_len;
 };
 
-enum wlr_color_transform_type {
-	COLOR_TRANSFORM_SRGB,
-	COLOR_TRANSFORM_LUT_3D,
-};
-
-struct wlr_color_transform {
-	int ref_count;
-	struct wlr_addon_set addons; // per-renderer helper state
-
-	enum wlr_color_transform_type type;
-	struct wlr_color_transform_lut3d lut3d;
-};
+/**
+ * Gets a wlr_color_transform_lut3d from a generic wlr_color_transform.
+ * Asserts that the base type is COLOR_TRANSFORM_LUT_3D
+ */
+struct wlr_color_transform_lut3d *wlr_color_transform_lut3d_from_base(
+	struct wlr_color_transform *tr);
 
 #endif