Add backward compatibility for AVIF chroma subsampling feature (#19698)

* AVIF export: add legacy_params for backward compatibility

Increment module version to 2 and add legacy_params function to
handle migration from version 1 (without subsample field) to
version 2 (with subsample field).

This ensures that existing AVIF export presets created before the
chroma subsampling feature continue to work after upgrade. Old
presets are automatically migrated with subsample set to AUTO,
preserving the original quality-based behavior.

Changes:
- Increment DT_MODULE from 1 to 2
- Add legacy_params() function to migrate v1 parameters to v2
- Old presets default to AVIF_SUBSAMPLE_AUTO for backward compatibility

Author: kadykov

src/imageio/format/avif.c

@@ -37,7 +37,7 @@
 #define AVIF_MAX_TILE_SIZE 3072
 #define AVIF_DEFAULT_TILE_SIZE AVIF_MIN_TILE_SIZE * 2
 
-DT_MODULE(1)
+DT_MODULE(2)
 
 enum avif_compression_type_e
 {
@@ -724,6 +724,48 @@ size_t params_size(dt_imageio_module_format_t *self)
   return sizeof(dt_imageio_avif_t);
 }
 
+void *legacy_params(dt_imageio_module_format_t *self,
+                    const void *const old_params,
+                    const size_t old_params_size,
+                    const int old_version,
+                    int *new_version,
+                    size_t *new_size)
+{
+  if(old_version == 1)
+  {
+    typedef struct dt_imageio_avif_v1_t
+    {
+      dt_imageio_module_data_t global;
+      uint32_t bit_depth;
+      uint32_t color_mode;
+      uint32_t compression_type;
+      uint32_t quality;
+      uint32_t tiling;
+    } dt_imageio_avif_v1_t;
+
+    if(old_params_size != sizeof(dt_imageio_avif_v1_t)) return NULL;
+
+    const dt_imageio_avif_v1_t *o = (dt_imageio_avif_v1_t *)old_params;
+    dt_imageio_avif_t *n = (dt_imageio_avif_t *)malloc(sizeof(dt_imageio_avif_t));
+    
+    if(!n) return NULL;
+
+    n->global = o->global;
+    n->bit_depth = o->bit_depth;
+    n->color_mode = o->color_mode;
+    n->compression_type = o->compression_type;
+    n->quality = o->quality;
+    n->tiling = o->tiling;
+    n->subsample = AVIF_SUBSAMPLE_AUTO; // Default to auto mode for old presets
+
+    *new_version = 2;
+    *new_size = sizeof(dt_imageio_avif_t);
+    return n;
+  }
+
+  return NULL;
+}
+
 void *get_params(dt_imageio_module_format_t *self)
 {
   dt_imageio_avif_t *d = calloc(1, sizeof(dt_imageio_avif_t));