### 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)

Vector<Vector2> point_list = {
		Vector2(3, (3.0 * (1.0 - p_overlap * 2.0)) / 2.0),
		Vector2(3, 3.0 * (1.0 - p_overlap * 2.0)),
		Vector2(2, 3.0 * (1.0 - (p_overlap * 2.0) * 2.0 / 3.0)),
		Vector2(1, 3.0 - p_overlap * 2.0),
		Vector2(0, 3),
		Vector2(-1, 3.0 - p_overlap * 2.0),
		Vector2(-2, 3.0 * (1.0 - (p_overlap * 2.0) * 2.0 / 3.0)),
		Vector2(-3, 3.0 * (1.0 - p_overlap * 2.0)),
		Vector2(-3, (3.0 * (1.0 - p_overlap * 2.0)) / 2.0),
		Vector2(-3, -(3.0 * (1.0 - p_overlap * 2.0)) / 2.0),
		Vector2(-3, -3.0 * (1.0 - p_overlap * 2.0)),
		Vector2(-2, -3.0 * (1.0 - (p_overlap * 2.0) * 2.0 / 3.0)),
		Vector2(-1, -(3.0 - p_overlap * 2.0)),