### 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)

BIND_ENUM_CONSTANT(TILE_LAYOUT_DIAMOND_RIGHT);
	BIND_ENUM_CONSTANT(TILE_LAYOUT_DIAMOND_DOWN);
	BIND_ENUM_CONSTANT(TILE_OFFSET_AXIS_HORIZONTAL);
	BIND_ENUM_CONSTANT(TILE_OFFSET_AXIS_VERTICAL);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_RIGHT_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_RIGHT_CORNER);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_BOTTOM_RIGHT_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_BOTTOM_RIGHT_CORNER);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_BOTTOM_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_BOTTOM_CORNER);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_BOTTOM_LEFT_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_BOTTOM_LEFT_CORNER);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_LEFT_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_LEFT_CORNER);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_TOP_LEFT_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_TOP_LEFT_CORNER);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_TOP_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_TOP_CORNER);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_TOP_RIGHT_SIDE);
	BIND_ENUM_CONSTANT(CELL_NEIGHBOR_TOP_RIGHT_CORNER);
	BIND_ENUM_CONSTANT(TERRAIN_MODE_MATCH_CORNERS_AND_SIDES);
	BIND_ENUM_CONSTANT(TERRAIN_MODE_MATCH_CORNERS);
	BIND_ENUM_CONSTANT(TERRAIN_MODE_MATCH_SIDES);
}
TileSet::TileSet() {
	
	tile_lines_mesh.instantiate();
	tile_filled_mesh.instantiate();
}
TileSet::~TileSet() {