fixes to the ImageSizer: cropBeforeResize, checkMemoryForImage, loadImageInfo

EDIT: this PR contains 4 commits!

1) https://github.com/horst-n/ProcessWire/commit/6640df43212620532bcd7c24c5936823178d3af6
When images gets cropped-before-resize and upscaling is not allowed and the resulting dimensions from crop-before-resize (cropExtra) are equal or smaller than the target-dimensions, the memory image wasn't saved to diskfile.

2) https://github.com/horst-n/ProcessWire/commit/11c9e0ca8a81ba17acca468b5ee999ed40531d75
refactored checkMemoryForImage, see following comment: https://github.com/ryancramerdesign/ProcessWire/pull/727#commitcomment-8213547

3) https://github.com/horst-n/ProcessWire/commit/d839615f9668f34def73a3e13acbd698efc6c4f9
made these vars readable too for the outside: cropExtra and options

4) https://github.com/horst-n/ProcessWire/commit/a578f0c14ee27c489576967fee22a1aa34657293
refactored loadImageInfo including one bug fix