default ffmpeg padding configuration is incompatible with ffmpeg2theora and the html5 theora preset

howdy!

i'm moving this configuration/documentation bug out of #989896: HTML5 Theora(ogv) (ffmpeg2theora) preset bug: --videobitrate parameter blank

on the transcoders settings page (admin/settings/video/transcoders) there is a setting named "FFMPeg Padding method"

the default value, "Use -padtop, -padbottom, -padleft, -padright for padding", is incompatible with ffmpeg2theora, and results in an error message if the "HTML5 Theora(ogv) (ffmpeg2theora)" preset is enabled

Video conversion failed for preset html5_ogv. FFMPEG reported the following output: Unknown preset....

ffmpeg2theora apparently interprets the -padtop parameter as a -p(reset) parameter and barfs.

ffmpeg2theora is only compatible with the "Use -vf "pad:w:h:x:y:c" for padding" setting.

since afaik -padtop is only provided for compatibility with older versions of ffmpeg, it would be better to make "Use -vf "pad:w:h:x:y:c" for padding" the default.

however, whichever default is chosen, the documentation should be updated to explain this incompatiblity.

i'm happy to provide a patch to change the default setting, if the maintainers would like one