Fix T44682: Save Buffers canceled renders show nothing in Image Editor

The issue was caused by render pipeline freeing render parts prior to finishing
exr file writing which resulted in unfinished parts not being written into the
file by save_empty_result_tiles().

As a temporary solution we do explicitly write unfinished parts as empty tiles
to the exr file prior to freeing parts.

Not ideal solution, but should work for the release.

source/blender/render/intern/include/render_result.h

@@ -79,6 +79,7 @@ void render_result_single_layer_end(struct Render *re);
 
 /* EXR Tile File Render */
 
+void render_result_save_empty_result_tiles(struct Render *re);
 void render_result_exr_file_begin(struct Render *re);
 void render_result_exr_file_end(struct Render *re);