### 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 (components.size() < 2) {
			return false;
		}
		String what = components[1];
		if (what == "name") {
			ctd->name = p_value;
		} else if (what == "texture") {
			ctd->texture = p_value;
		} else if (what == "tex_offset") {
			ctd->tex_offset = p_value;
		} else if (what == "material") {
			ctd->material = p_value;
		} else if (what == "modulate") {
			ctd->modulate = p_value;
		} else if (what == "region") {
			ctd->region = p_value;
		} else if (what == "tile_mode") {
			ctd->tile_mode = p_value;
		} else if (what.left(9) == "autotile") {
			what = what.substr(9);
			if (what == "bitmask_mode") {
				ctd->autotile_bitmask_mode = p_value;
			} else if (what == "icon_coordinate") {
				ctd->autotile_icon_coordinate = p_value;
			} else if (what == "tile_size") {
				ctd->autotile_tile_size = p_value;
			} else if (what == "spacing") {
				ctd->autotile_spacing = p_value;
			} else if (what == "bitmask_flags") {
				if (p_value.is_array()) {