Fixed bugs with fail at crop mode and incorrect aspect in crop and padding mode

Crop bug:
When using 'mode': 'crop', converter fails with message: "converter.ffmpeg.FFMpegConvertError: Unknown ffmpeg error". It is because avcodecs.py generates to ffmpeg "-vf crop=crop-left:crop-up:x-size:y-size", but according https://bbs.archlinux.org/viewtopic.php?pid=1147161#p1147161 it must generate "-vf crop=x-size:y-size:crop-left:crop-up". Fixed in this pull.
Aspect bug:
For 320x240 video from widescreen it generates "-s 320x179 -aspect 320:179 -vf pad=320:240:0:30". It forces video players use incorrect aspect for converted video. Workaround: specify aspect manually in video player settings. It works, but it is bad solution. Actually avcodecs.py must generate "-s 320x179 -aspect 320:240 -vf pad=320:240:0:30" for such case. Fixed in this pull.