Prohibit zero-sized pixmaps

[LaurenzV]

Currently, it is possible to create pixmaps where the width or height is 0. This can lead to panics when we start sampling the image when rendering. We probably should prohibit the user from creating such pixmaps.

[nicoburns]

Hmm.. It seems useful to be able to create this as a no-allocation state that avoids the need for an Option. Could we just add a pre-render check and return an Err if it's zero-sized (I think in Winit-based apps it's common not to have the size available when you initialize state, but you get sent it before you actually render).