changed to set scaleMethod option only for non-truecolor image (e.g. well-used gif images) (refs #3677, BP from #3062)

Conflicts:
	lib/plugins/sfImageHandlerPlugin/lib/image/generator/sfImageGeneratorImageTransform.php
changed not to call crop() method twice (refs #3677, BP from #3062)
Merge remote-tracking branch 'ama/t-3677' into stable-3.8.x