Avoid unwraps on pixmap creation

[Its-Just-Nans]

Fix #853

[LaurenzV]

Ah, it's probably because the maximum size isn't actually 32-bit, but 31. Then this change makes sense, but we need to remove the comments.

[Its-Just-Nans]

Ah, it's probably because the maximum size isn't actually 32-bit, but 31. Then this change makes sense, but we need to remove the comments.

Yes Pixmap's width is limited by i32::MAX/4, we handle the creation error.

Also I think this MR linebender/tiny-skia#151 could help to handle panic (Out Of Memory) in this case

[LaurenzV]

Thanks!