Shape Keys: replace the BKE_keyblock_from_key function with find_index.

The function for retrieving a shape key by its index is named
somewhat confusingly, and effectively reimplements BLI_findlink.

However, more importantly, for some reason it is coded to return
null for the index 0 instead of the basis shape key. This severely
limits its usability in some cases.

This refactor replaces the function with a simple strongly typed
wrapper around BLI_findlink, using a different name, and updating
the call sites to check that the index is not 0 where necessary.

source/blender/blenkernel/BKE_key.h

@@ -88,9 +88,9 @@ struct KeyBlock *BKE_keyblock_add(struct Key *key, const char *name);
  */
 struct KeyBlock *BKE_keyblock_add_ctime(struct Key *key, const char *name, bool do_force);
 /**
- * Get the appropriate #KeyBlock given an index.
+ * Get the appropriate #KeyBlock given an index (0 refers to the basis key). Key may be null.
  */
-struct KeyBlock *BKE_keyblock_from_key(struct Key *key, int index);
+struct KeyBlock *BKE_keyblock_find_by_index(struct Key *key, int index);
 /**
  * Get the appropriate #KeyBlock given a name to search for.
  */

source/blender/blenkernel/intern/DerivedMesh.cc

@@ -357,9 +357,9 @@ static float (*get_orco_coords(Object *ob, BMEditMesh *em, int layer, int *free)
     if (!em) {
       ClothModifierData *clmd = (ClothModifierData *)BKE_modifiers_findby_type(
           ob, eModifierType_Cloth);
-      if (clmd) {
-        KeyBlock *kb = BKE_keyblock_from_key(BKE_key_from_object(ob),
-                                             clmd->sim_parms->shapekey_rest);
+      if (clmd && clmd->sim_parms->shapekey_rest) {
+        KeyBlock *kb = BKE_keyblock_find_by_index(BKE_key_from_object(ob),
+                                                  clmd->sim_parms->shapekey_rest);
 
         if (kb && kb->data) {
           return (float(*)[3])kb->data;

source/blender/blenkernel/intern/ipo.cc

@@ -443,7 +443,7 @@ static char *shapekey_adrcodes_to_paths(ID *id, int adrcode, int * /*r_array_ind
   else {
     /* Find the name of the ShapeKey (i.e. KeyBlock) to look for */
     Key *key = (Key *)id;
-    KeyBlock *kb = BKE_keyblock_from_key(key, adrcode);
+    KeyBlock *kb = BKE_keyblock_find_by_index(key, adrcode);
 
     /* setting that we alter is the "value" (i.e. keyblock.curval) */
     if (kb) {

source/blender/blenkernel/intern/key.cc

@@ -1897,12 +1897,7 @@ KeyBlock *BKE_keyblock_from_object(Object *ob)
 {
   Key *key = BKE_key_from_object(ob);
 
-  if (key) {
-    KeyBlock *kb = static_cast<KeyBlock *>(BLI_findlink(&key->block, ob->shapenr - 1));
-    return kb;
-  }
-
-  return nullptr;
+  return BKE_keyblock_find_by_index(key, ob->shapenr - 1);
 }
 
 KeyBlock *BKE_keyblock_from_object_reference(Object *ob)
@@ -1916,21 +1911,13 @@ KeyBlock *BKE_keyblock_from_object_reference(Object *ob)
   return nullptr;
 }
 
-KeyBlock *BKE_keyblock_from_key(Key *key, int index)
+KeyBlock *BKE_keyblock_find_by_index(Key *key, int index)
 {
-  if (key) {
-    KeyBlock *kb = static_cast<KeyBlock *>(key->block.first);
-
-    for (int i = 1; i < key->totkey; i++) {
-      kb = kb->next;
-
-      if (index == i) {
-        return kb;
-      }
-    }
+  if (!key) {
+    return nullptr;
   }
 
-  return nullptr;
+  return static_cast<KeyBlock *>(BLI_findlink(&key->block, index));
 }
 
 KeyBlock *BKE_keyblock_find_name(Key *key, const char name[])

source/blender/modifiers/intern/MOD_cloth.cc

@@ -100,8 +100,8 @@ static void deform_verts(ModifierData *md,
    * Also hopefully new cloth system will arrive soon..
    */
   if (mesh == nullptr && clmd->sim_parms->shapekey_rest) {
-    KeyBlock *kb = BKE_keyblock_from_key(BKE_key_from_object(ctx->object),
-                                         clmd->sim_parms->shapekey_rest);
+    KeyBlock *kb = BKE_keyblock_find_by_index(BKE_key_from_object(ctx->object),
+                                              clmd->sim_parms->shapekey_rest);
     if (kb && kb->data != nullptr) {
       float(*layerorco)[3];
       if (!(layerorco = static_cast<float(*)[3]>(