Screenshot images are skewed
Description of problem:
1. Start a guest with SPICE
2. Connect with a SPICE client
3. Resize screen to a width that is not a multiple of 4 (e. g. 487x956)
4. Take a screenshot

The screenshot ppm file will contain the actual dimensions in the header, e. g.
```
P6
487 956
255
```
but the image data will contain more than that (e. g. 488 * 956 * 3 bytes).
As a result, when displaying the image it appears skewed.
Steps to reproduce:
See above.
Additional information:
I'm not familiar with qemu code nor the pixman library, but I assume that in [this line](https://gitlab.com/qemu-project/qemu/-/blob/bc6ec396d471d9e4aae7e2ff8b72e11da9a97665/ui/console.c#L316) `get_stride` is wrong. Instead, it should write `width*3` bytes.