Fix: incorrect GPU ownership after IMB_dupImBuf

The IMB_dupImBuf() function does not copy the GPU texture as it might
not be possible if the GPU context is not active, but it also was not
clearing the pointer in the result ImBuf. This could potentially lead
to situation when the texture gets freed via IMB_freeImBuf of either
original or copied image buffer, leaving the other pointing to a freed
GPU texture.

It is not known whether it ever was an actual problem for artists,
but it is nice to avoid such possibility.

Pull Request: https://projects.blender.org/blender/blender/pulls/119469

source/blender/imbuf/intern/allocimbuf.cc

@@ -635,6 +635,10 @@ ImBuf *IMB_dupImBuf(const ImBuf *ibuf1)
   tbuf.display_buffer_flags = nullptr;
   tbuf.colormanage_cache = nullptr;
 
+  /* GPU textures can not be easily copied, as it is not guaranteed that this function is called
+   * from within an active GPU context. */
+  tbuf.gpu.texture = nullptr;
+
   *ibuf2 = tbuf;
 
   return ibuf2;