### Godot version

v4.2.dev3.official [013e8e3af]

### System information

Windows 11, Core i5-12400F, RX 6600

### Issue description

Any images without transparency become RGB8.
Any images with transparency become RGBA8.

When doing a TileSet Atlas Merge RGB8 images do not show up in merge dialog (just empty space) and do not merge.

![image](https://github.com/godotengine/godot/assets/962340/9cd422ce-2f86-41aa-b4f1-97bcba2db003)![Screenshot 2023-08-21 203922](https://github.com/godotengine/godot/assets/962340/905fe87d-c641-4c6e-bd0e-c47fe997c2ff)


### Steps to reproduce

- merge any solid image with no transparency

### Minimal reproduction project

https://github.com/rakkarage/TestTestTest333

EDIT: Added simpler example Test2.tscn, without margin, separation, and using real default size tiles...
#include "atlas_merging_dialog.h"
#include "editor/editor_properties_vector.h"
#include "editor/editor_scale.h"
#include "editor/editor_undo_redo_manager.h"
#include "editor/gui/editor_file_dialog.h"
#include "scene/gui/control.h"
#include "scene/gui/split_container.h"
#include "scene/resources/image_texture.h"
void AtlasMergingDialog::_property_changed(const StringName &p_property, const Variant &p_value, const String &p_field, bool p_changing) {