Fix some -Wshadow warnings

In general, this warning can hint to actual bugs. We don't enable it
yet, because it would conflict with some unmerged code, and we should
check with clang too (this commit was done by testing with gcc).
mplayer: remove dead code

Attached pictures are now handled specially and on a separate codepath,
so this hack is not needed anymore. Remove it from the normal codepath.
demux_lavf: set keyframe flag on attached pictures

Gives really funky results with PNG attachments otherwise. The main
problem is that avcodec_flush_buffers() does not fully reset the
decoder, so passing multiple PNG packets without keyframe flags will
attempt to combine the new picture with the previously decoded
contents. (Makes no sense with proper PNG - maybe this codepath is
intended for MNG or APNG.)
input: fix dangling pointer issues with multi-part commands

This didn't setup the linked list of sub-commands for multi-part
commands ("a ; b ; c") correctly. Also, the new commands were attached
to the allocation of the _old_ command instead of the new one. (Wow,
whatever the hell I was (not) thinking when I wrote this code.)
m_config: don't override profile option if not required

The --profile top-level option is handled specially in m_config.c. But
this code also broke sub-options that happened to be named "profile"
(e.g. when trying to use "-af-add=bs2b=profile=cmoy"). Handle it
specially only if it's the top-level --profile option.
mp_image: create AVBuffers for all planes when converting to AVFrame

It appears the API requires you to cover all plane data with AVBuffers
(that is, one AVBuffer per plane in the most general case), because
certain code can make certain assumptions about this. (Insert rant
about how this is barely useful and increases complexity and potential
bugs.) I don't know any cases where the current code actually fails,
but we want to follow the API, so do it anyway.

Note that we don't really know whether or not planes are from a single
memory allocation, so we have to assume the most general case and create
an AVBuffer for each plane. We simply assume that the data is padded to
the full stride in the last image line. All these extra dummy references
are stupid, but the code might become much simpler once we only support
libavcodec versions with refcounting and can use AVFrame directly.
demux_libass: actually set priv context

This was a memory leak: the ASS_Track was never deallocated.