Refactor: initialize IMB after parsing settings

Avoid awkward kludges with IMB_init/exit & IMB_ffmpeg_init calls.

The animation player was responsible for calling IMB_exit, not IMB_init,
this complicated drag & drop which needed to re-internalize IMB before
returning.

Delay initializing IMB since none of the other argument handlers
depend on it, make wm_main_playanim_intern responsible for calling
both initialization & exit calls for IMB.

Also expand code comments for ARG_PASS_SETTINGS.

source/blender/windowmanager/intern/wm_playanim.cc

@@ -1712,6 +1712,11 @@ static bool wm_main_playanim_intern(int argc, const char **argv, PlayArgs *args_
 
   ps.font_id = -1;
 
+  IMB_init();
+#ifdef WITH_FFMPEG
+  IMB_ffmpeg_init();
+#endif
+
   STRNCPY(ps.display_ctx.display_settings.display_device,
           IMB_colormanagement_role_colorspace_name_get(COLOR_ROLE_DEFAULT_BYTE));
   IMB_colormanagement_init_default_view_settings(&ps.display_ctx.view_settings,
@@ -2145,7 +2150,8 @@ static bool wm_main_playanim_intern(int argc, const char **argv, PlayArgs *args_
   BLF_exit();
 
   /* NOTE: Must happen before GPU Context destruction as GPU resources are released via
-   * Color Management module. Must be re-initialized in the case of drag & drop. */
+   * Color Management module.
+   * NOTE: there is no #IMB_ffmpeg_exit. */
   IMB_exit();
 
   if (ps.ghost_data.gpu_context) {
@@ -2161,9 +2167,6 @@ static bool wm_main_playanim_intern(int argc, const char **argv, PlayArgs *args_
 
   /* Early exit, IMB and BKE should be exited only in end. */
   if (ps.argv_next) {
-    /* Ensure drag & drop runs with a valid IMB state. */
-    IMB_init();
-
     args_next->argc = ps.argc_next;
     args_next->argv = ps.argv_next;
     return true;

source/creator/creator.cc

@@ -486,16 +486,23 @@ int main(int argc,
   /* After parsing number of threads argument. */
   BLI_task_scheduler_init();
 
-  /* Initialize sub-systems that use `BKE_appdir.h`. */
-  IMB_init();
-
 #ifndef WITH_PYTHON_MODULE
-  /* First test for background-mode (#Global.background) */
+  /* The settings pass includes:
+   * - Background-mode assignment (#Global.background), checked by other subsystems
+   *   which may be skipped in background mode.
+   * - The animation player may be launched which takes over argument passing,
+   *   initializes the sub-systems it needs which have not yet been started.
+   *   The animation player will call `exit(..)` too, so code after this call
+   *   never runs when it's invoked.
+   * - All the `--debug-*` flags.
+   */
   BLI_args_parse(ba, ARG_PASS_SETTINGS, nullptr, nullptr);
 
   main_signal_setup();
 #endif
 
+  /* Must be initialized after #BKE_appdir_init to account for color-management paths. */
+  IMB_init();
 #ifdef WITH_FFMPEG
   /* Keep after #ARG_PASS_SETTINGS since debug flags are checked. */
   IMB_ffmpeg_init();

source/creator/creator_args.cc

@@ -1389,11 +1389,6 @@ static int arg_handle_playback_mode(int argc, const char **argv, void * /*data*/
 {
   /* Ignore the animation player if `-b` was given first. */
   if (G.background == 0) {
-#  ifdef WITH_FFMPEG
-    /* Setup FFmpeg with current debug flags. */
-    IMB_ffmpeg_init();
-#  endif
-
     /* Skip this argument (`-a`). */
     WM_main_playanim(argc - 1, argv + 1);