lavfi: add const to AVFilterContext.filter.

lavfi should never modify the filter through that pointer.
lavfi: add const to the pads parameter of avfilter_pad_get_name/type
lavfi: merge avfiltergraph.h into avfilter.h

We do not support using filters without AVFilterGraph in practice
anyway, so there is no point in pretending we do.
lavfi: replace avfilter_open() with avfilter_graph_alloc_filter().

Since we do not support "standalone" filters not attached to an
AVFilterGraph, we should not have a public function to create such
filters. In addition that function is horribly named, the action it does
cannot be possibly described as "opening" a filter.
lavfi: deprecate avfilter_graph_add_filter().

Since this function adds a standalone filter to a filter graph and we do
not support creating such filters, there is no reason for this function
to exist.
lavfi: add AVFilterContext.graph.

It will be useful in the following commits.
lavfi: make avfilter_free() remove the filter from its graph.
lavfi: add avfilter_init_str() to replace avfilter_init_filter().

Drop the unused opaque parameter from its signature.
lavfi/mp: switch to an AVOptions-based system.
Merge commit '4a37d4b3f8137a4c2bbbca043de076af966b9446'

* commit '4a37d4b3f8137a4c2bbbca043de076af966b9446':
  lavfi: add const to the pads parameter of avfilter_pad_get_name/type
  lavfi: add const to AVFilterContext.filter.

Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/filters: use -af and -filter_complex instead of amovie for two examples.
Merge commit '38f0c0781a6e099f11c0acec07f9b8be742190c4'

* commit '38f0c0781a6e099f11c0acec07f9b8be742190c4':
  lavfi: merge avfiltergraph.h into avfilter.h

Conflicts:
	doc/APIchanges
	ffmpeg_filter.c
	libavfilter/avfilter.h
	libavfilter/avfiltergraph.h
	libavfilter/version.h
	tools/graph2dot.c

Merged-by: Michael Niedermayer <michaelni@gmx.at>
lavfi/overlay: reindent constants in option list

Wanted-By: ubitux
lavfi/overlay: add repeatlast option
lavfi/overlay: fix crash in case of invalid expression
doc/filters: apply minor fixes to hue Commands section
doc/filters: rework Commands section for overlay filter
lavfi/overlay: improve feedback in case of invalid expression

Based on vf_hue.c code.
Merge commit 'bc1a985ba030e9861d24965d42792850b43a43ea'

* commit 'bc1a985ba030e9861d24965d42792850b43a43ea':
  lavfi: replace avfilter_open() with avfilter_graph_alloc_filter().

Conflicts:
	libavfilter/avfiltergraph.c
	libavfilter/internal.h
	libavfilter/version.h

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'c2c9801bc9bce688d51d1a96f5f3ea93933e2dee'

* commit 'c2c9801bc9bce688d51d1a96f5f3ea93933e2dee':
  lavfi: deprecate avfilter_graph_add_filter().

Merged-by: Michael Niedermayer <michaelni@gmx.at>