libhb: make hb_get_preview2 always return a valid image
When we fail to read the preview image, create a blank image


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6612 b64f7644-9d1e-0410-96f1-a4d463321fa5
libhb: add hb_image_init to create blank images


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6613 b64f7644-9d1e-0410-96f1-a4d463321fa5
json: fix hb_get_preview_json
and add some routines for json conversion of preview parameters and images


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6614 b64f7644-9d1e-0410-96f1-a4d463321fa5