Generalized core:image loader

```odin
import "core:image"
import "core:image/png"

...

img, err := image.load_from_file("path.png")
```

core/image/qoi/qoi.odin

@@ -403,4 +403,9 @@ qoi_hash :: #force_inline proc(pixel: RGBA_Pixel) -> (index: u8) {
 	i4 := u16(pixel.a) * 11
 
 	return u8((i1 + i2 + i3 + i4) & 63)
+}
+
+@(init, private)
+_register :: proc() {
+	image.register_loader(.QOI, load_from_slice)
 }