### 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)

if (tile_shape == TileSet::TILE_SHAPE_SQUARE) {
		if (p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS_AND_SIDES || p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_SIDES) {
			if (p_peering_bit == TileSet::CELL_NEIGHBOR_RIGHT_SIDE ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_BOTTOM_SIDE ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_LEFT_SIDE ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_TOP_SIDE) {
				return true;
			}
		}
		if (p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS_AND_SIDES || p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS) {
			if (p_peering_bit == TileSet::CELL_NEIGHBOR_BOTTOM_RIGHT_CORNER ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_BOTTOM_LEFT_CORNER ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_TOP_LEFT_CORNER ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_TOP_RIGHT_CORNER) {
				return true;
			}
		}
	} else if (tile_shape == TileSet::TILE_SHAPE_ISOMETRIC) {
		if (p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS_AND_SIDES || p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_SIDES) {
			if (p_peering_bit == TileSet::CELL_NEIGHBOR_BOTTOM_RIGHT_SIDE ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_BOTTOM_LEFT_SIDE ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_TOP_LEFT_SIDE ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_TOP_RIGHT_SIDE) {
				return true;
			}
		}
		if (p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS_AND_SIDES || p_terrain_mode == TileSet::TERRAIN_MODE_MATCH_CORNERS) {
			if (p_peering_bit == TileSet::CELL_NEIGHBOR_RIGHT_CORNER ||
					p_peering_bit == TileSet::CELL_NEIGHBOR_BOTTOM_CORNER ||