### Godot version

5ee983188de97ae027f9b9c1443438063f708a7e

### System information

Windows 11

### Issue description

Godot editor will throw an error and fail to generate preview icons for terrain tiles which use a base image which is not RGBA8 format.
`core\io\image.cpp:2788 - Condition "format != p_src->format" is true.`

### Steps to reproduce

- Import a non-RGBA8 image into Godot.
- Create a TileMap
- Create an associated TileSet
- Create a terrain for this tileset
- Add the image as an atlas
- Go to the paint tab
- Paint the terrain on the TileSet
- An error will occur any time terrains are edited in the TileSet editor or selected in the TileMap editor terrain tab.

### Minimal reproduction project

[tileset_terrain_bug.zip](https://github.com/godotengine/godot/files/13264213/tileset_terrain_bug.zip)

ERR_FAIL_COND_V(p_margins.x < 0 || p_margins.y < 0, PackedVector2Array());
	ERR_FAIL_COND_V(p_separation.x < 0 || p_separation.y < 0, PackedVector2Array());
	ERR_FAIL_COND_V(p_texture_region_size.x <= 0 || p_texture_region_size.y <= 0, PackedVector2Array());
	
	Size2 new_grid_size;
	if (p_texture.is_valid()) {
		Size2i valid_area = p_texture->get_size() - p_margins;
		
		if (valid_area.x >= p_texture_region_size.x && valid_area.y >= p_texture_region_size.y) {
			valid_area -= p_texture_region_size;
			new_grid_size = Size2i(1, 1) + valid_area / (p_texture_region_size + p_separation);
		}
	}
	Vector<Vector2> output;
	for (KeyValue<Vector2i, TileAlternativesData> &E : tiles) {
		for (unsigned int frame = 0; frame < E.value.animation_frames_durations.size(); frame++) {
			Vector2i frame_coords = E.key + (E.value.size_in_atlas + E.value.animation_separation) * ((E.value.animation_columns > 0) ? Vector2i(frame % E.value.animation_columns, frame / E.value.animation_columns) : Vector2i(frame, 0));
			frame_coords += E.value.size_in_atlas;
			if (frame_coords.x > new_grid_size.x || frame_coords.y > new_grid_size.y) {
				output.push_back(E.key);
				break;
			}
		}
	}
	return output;
}
Rect2i TileSetAtlasSource::get_tile_texture_region(Vector2i p_atlas_coords, int p_frame) const {