### 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)

overlap = 0.25;
				break;
			case TileSet::TILE_SHAPE_HALF_OFFSET_SQUARE:
				overlap = 0.0;
				break;
			default:
				break;
		}
		if (terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS_AND_SIDES) {
			return _get_half_offset_corner_or_side_terrain_peering_bit_polygon(tile_size, overlap, tile_offset_axis, p_bit);
		} else if (terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS) {
			return _get_half_offset_corner_terrain_peering_bit_polygon(tile_size, overlap, tile_offset_axis, p_bit);
		} else { 
			return _get_half_offset_side_terrain_peering_bit_polygon(tile_size, overlap, tile_offset_axis, p_bit);
		}
	}
}
#define TERRAIN_ALPHA 0.6
void TileSet::draw_terrains(CanvasItem *p_canvas_item, Transform2D p_transform, const TileData *p_tile_data) {
	ERR_FAIL_NULL(p_tile_data);
	if (terrain_bits_meshes_dirty) {
		
		terrain_peering_bits_meshes.clear();
		for (int terrain_mode_index = 0; terrain_mode_index < 3; terrain_mode_index++) {
			TerrainMode terrain_mode = TerrainMode(terrain_mode_index);
			
			Vector<Vector2> polygon;
			if (tile_shape == TileSet::TILE_SHAPE_SQUARE) {
				polygon = _get_square_terrain_polygon(tile_size);
			} else if (tile_shape == TileSet::TILE_SHAPE_ISOMETRIC) {