Fix (unreported) crash in blendkernel unittest in debug builds.

Some `ImagePartialUpdateTest` test are calling code that needs access to
a valid `G_MAIN`. So store the generated main there as part of the setup
step, and reset G_MAIN to its original value (should be NULL) in the
teardown step.

NOTE: Things like `ID_BLEND_PATH_FROM_GLOBAL` and
`BKE_main_blendfile_path_from_global` are pure evil. It may be necessary
in a very few small cases, but their current usages need a lot of strong
cleanup.

Pull Request: https://projects.blender.org/blender/blender/pulls/108189

source/blender/blenkernel/intern/image_partial_update_test.cc

@@ -5,6 +5,7 @@
 #include "CLG_log.h"
 
 #include "BKE_appdir.h"
+#include "BKE_global.h"
 #include "BKE_idtype.h"
 #include "BKE_image.h"
 #include "BKE_image_partial_update.hh"
@@ -24,6 +25,7 @@ constexpr float black_color[4] = {0.0f, 0.0f, 0.0f, 1.0f};
 class ImagePartialUpdateTest : public testing::Test {
  protected:
   Main *bmain;
+  Main *prev_bmain;
   Image *image;
   ImageTile *image_tile;
   ImageUser image_user = {nullptr};
@@ -55,6 +57,9 @@ class ImagePartialUpdateTest : public testing::Test {
     IMB_init();
 
     bmain = BKE_main_new();
+    /* Required by usage of #ID_BLEND_PATH_FROM_GLOBAL in #add_ibuf_for_tile. */
+    prev_bmain = G_MAIN;
+    G_MAIN = bmain;
     /* Creating an image generates a memory-leak during tests. */
     image = create_test_image(1024, 1024);
     image_tile = BKE_image_get_tile(image, 0);
@@ -67,6 +72,9 @@ class ImagePartialUpdateTest : public testing::Test {
   {
     BKE_image_release_ibuf(image, image_buffer, nullptr);
     BKE_image_partial_update_free(partial_update_user);
+
+    /* Restore original main in G_MAIN. */
+    G_MAIN = prev_bmain;
     BKE_main_free(bmain);
 
     IMB_moviecache_destruct();