UDIM: Support virtual filenames

This implements the design detailed in T92696 to support virtual
filenames for UDIM textures. Currently, the following 2 substitution
tokens are supported:

| Token | Meaning |
| ----- | ---- |
| <UDIM>   | 1001 + u-tile + v-tile * 10 |
| <UVTILE> | Equivalent to u<u-tile + 1>_v<v-tile + 1> |

Example for u-tile of 3 and v-tile of 1:
filename.<UDIM>_ver0023.png   --> filename.1014_ver0023.png
filename.<UVTILE>_ver0023.png --> filename.u4_v2_ver0023.png

For image loading, the existing workflow is unchanged. A user can select
one or more image files, belonging to one or more UDIM tile sets, and
have Blender load them all as it does today. Now the <UVTILE> format is
"guessed" just as the <UDIM> format was guessed before.

If guessing fails, the user can simply go into the Image Editor and type
the proper substitution in the filename. Once typing is complete,
Blender will reload the files and correctly fill the tiles. This
workflow is new as attempting to fix the guessing in current versions
did not really work, and the user was often stuck with a confusing
situation.

For image saving, the existing workflow is changed slightly. Currently,
when saving, a user has to be sure to type the filename of the first
tile (e.g. filename.1001.png) to save the entire UDIM set. The number
could differ if they start at a different tile etc. This is confusing.
Now, the user should type a filename containing the appropriate
substitution token. By default Blender will fill in a default name using
the <UDIM> token but the user is free to save out images using <UVTILE>
if they wish.

Differential Revision: https://developer.blender.org/D13057

source/blender/blenkernel/intern/bpath.c

@@ -236,7 +236,8 @@ void BKE_bpath_missing_files_check(Main *bmain, ReportList *reports)
   BKE_bpath_foreach_path_main(&(BPathForeachPathData){
       .bmain = bmain,
       .callback_function = check_missing_files_foreach_path_cb,
-      .flag = BKE_BPATH_FOREACH_PATH_ABSOLUTE | BKE_BPATH_FOREACH_PATH_SKIP_PACKED,
+      .flag = BKE_BPATH_FOREACH_PATH_ABSOLUTE | BKE_BPATH_FOREACH_PATH_SKIP_PACKED |
+              BKE_BPATH_FOREACH_PATH_RESOLVE_TOKEN,
       .user_data = reports});
 }
 
@@ -384,7 +385,8 @@ void BKE_bpath_missing_files_find(Main *bmain,
                                   const bool find_all)
 {
   struct BPathFind_Data data = {NULL};
-  const int flag = BKE_BPATH_FOREACH_PATH_ABSOLUTE | BKE_BPATH_FOREACH_PATH_RELOAD_EDITED;
+  const int flag = BKE_BPATH_FOREACH_PATH_ABSOLUTE | BKE_BPATH_FOREACH_PATH_RELOAD_EDITED |
+                   BKE_BPATH_FOREACH_PATH_RESOLVE_TOKEN;
 
   data.basedir = BKE_main_blendfile_path(bmain);
   data.reports = reports;