### 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...
if (frame > 0) {
						new_position += src_rect.size * Vector2i(frame, 0);
						atlas_size.x = MAX(frame + 1, atlas_size.x);
					}
					Rect2 dst_rect_wide = Rect2i(new_position, new_tile_rect_in_atlas.size * new_texture_region_size);
					if (dst_rect_wide.get_end().x > output_image->get_width() || dst_rect_wide.get_end().y > output_image->get_height()) {
						output_image->crop(MAX(dst_rect_wide.get_end().x, output_image->get_width()), MAX(dst_rect_wide.get_end().y, output_image->get_height()));
					}
					output_image->blit_rect(atlas_source->get_texture()->get_image(), src_rect, dst_rect_wide.get_center() - src_rect.size / 2);
				}
				
				merged_mapping[source_index][tile_id] = new_tile_rect_in_atlas.position;
			}
			
			line_height = MAX(atlas_size.y, line_height);
			atlas_offset.x += atlas_size.x;
			if (atlas_offset.x >= p_max_columns) {
				atlas_offset.x = 0;
				atlas_offset.y += line_height;
				line_height = 0;
			}
		}
		merged->set_name(p_atlas_sources[0]->get_name());
		merged->set_texture(ImageTexture::create_from_image(output_image));
		merged->set_texture_region_size(new_texture_region_size);
		
		for (int source_index = 0; source_index < p_atlas_sources.size(); source_index++) {
			Ref<TileSetAtlasSource> atlas_source = p_atlas_sources[source_index];
			for (KeyValue<Vector2i, Vector2i> tile_mapping : merged_mapping[source_index]) {