Address performance penalty when source is smaller than one or more requested sizes

[rbuchberger]

Currently, if a source image is smaller than one of the requested images, we start the "generate a new image" process on every site build. Only after calculating a hash do we check sizes and discover the source is too small. The result is a moderate performance penalty for repeated builds on sites with many such images, even with fast_build enabled.

Solution: Check the image size first, then calculate a hash.

[Syonyk]

Following, this definitely seems to add some time to my blog renders.