bitmaptools.alphablend is wonky for some blend factors

### CircuitPython version

```python
latest
```


### Code/REPL

```python
bitmaptools.alphablend(dest, src1, src1, colorspace, factor1=0)
```


### Behavior

Result is incorrectly mixed

### Description

_No response_

### Additional information

![image](https://user-images.githubusercontent.com/1517291/141210107-67a3d173-0b7c-4224-8a7e-763701ddf61f.png)
