Warn or fail when using an incompatible input image size

[mortenjust]

When you pass an input image to the generator, it currently doesn't check if the image fits the model (currently we only support SD1.5, so it's always 512x512)

This warning is for all users using img2img via an inputImage

The warning could be an assertion failure, but open to ideas.

Next: Resize off-size images, using a center crop. Feel free to go for this directly. Later: Support for SD2.0