Fix #117539: Image.scale() always works on the first tile

`BKE_image_scale` -- which is only used for the python API -- was
getting the `ImBuf` without providing an `ImageUser`.
This is fine, but always gets the first tile.

In order to respect `Image.tiles.active_index` for the scaling
operation, we can create a default `ImageUser` and set its `tile`.

Afaict, no other members of the `ImageUser` need to be set for tiled
images, so should be no other functional changes (compared to passing a
nullptr).

@@ -184,1 +186,3 @@
-  if (!BKE_image_scale(image, width, height)) {
+  ImageUser iuser{};
+  BKE_imageuser_default(&iuser);
+  bool use_imageuser = false;

@@ -185,0 +208,4 @@
+    /* Not exactly sure if we should check this beforehand? */
+    char filepath[FILE_MAX];
+    BKE_image_user_file_path(&iuser, image, filepath);
+    if (BLI_open(filepath, O_BINARY | O_RDONLY, 0) == -1) {

@@ -185,0 +218,4 @@
+    }
+
+    use_imageuser = true;
+  }