Image.blt causes infinite loop when self-referenced
**Pyxel v 1.5.0**

Consider this code
```
pyxel.image(0).blt(0, 0, 1, 5, 7, 10, 10)
```
It works fine, but this code
```
pyxel.image(0).blt(0, 0, 0, 5, 7, 10, 10)
```
goes into infinite loop and game hangs. I'd expect it to be equivalent to
```py
for y in range(10):
  for x in range(10):
    image(0).pset(x, y, image(0).pget(x+5, y+7))
```