ff_filter_get_nb_threads() respect AVFilterContext.nb_threads and
graph->nb_threads both, in most case, we perfer this API than using
ctx->graph->nb_threads directly.

Reviewed-by: Steven Liu <lq@onvideo.cn>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

The lensfun filter wraps the lensfun library which performs
transformations on videos to correct for lens distortion. Often this
results in areas in the input being mapped to areas that fall outside
the boundaries of the output. The library has a parameter called scale
which is a scale factor applied to the output video. By decreasing it it
is possible to regain the areas of the video which would otherwise have
been lost. There is a special value of 0 which indicates that the
library should automatically determine a scale factor that results in
the output frame being filled (i.e. little or no black/unmapped areas).

This patch adds a corresponding scale option to the lensfun filter which
is passed through to the library. The existing behaviour of using the
automatic value of 0 is retained as the default behaviour, while other
values will be passed through to the library.

Signed-off-by: Daniel Playfair Cal <daniel.playfair.cal@gmail.com>

Lensfun is a library that applies lens correction to an image using a
database of cameras/lenses (you provide the camera and lens models, and
it uses the corresponding database entry's parameters to apply lens
correction). It is licensed under LGPL3.

The lensfun filter utilizes the lensfun library to apply lens
correction to videos as well as images.

This filter was created out of necessity since I wanted to apply lens
correction to a video and the lenscorrection filter did not work for me.

While this filter requires little info from the user to apply lens
correction, the flaw is that lensfun is intended to be used on indvidual
images. When used on a video, the parameters such as focal length is
constant, so lens correction may fail on videos where the camera's focal
length changes (zooming in or out via zoom lens). To use this filter
correctly on videos where such parameters change, timeline editing may
be used since this filter supports it.

Note that valgrind shows a small memory leak which is not from this
filter but from the lensfun library (memory is allocated when loading
the lensfun database but it somehow isn't deallocated even during
cleanup; it is briefly created in the init function of the filter, and
destroyed before the init function returns). This may have been fixed by
the latest commit in the lensfun repository; the current latest release
of lensfun is almost 3 years ago.

Bi-Linear interpolation is used by default as lanczos interpolation
shows more artifacts in the corrected image in my tests.

The lanczos interpolation is derived from lenstool's implementation of
lanczos interpolation. Lenstool is an app within the lensfun repository
which is licensed under GPL3.

v2 of this patch fixes license notice in libavfilter/vf_lensfun.c

v3 of this patch fixes code style and dependency to gplv3 (thanks to
Paul B Mahol for pointing out the mentioned issues).

v4 of this patch fixes more code style issues that were missed in
v3.

v5 of this patch adds line breaks to some of the documentation in
doc/filters.texi (thanks to Gyan Doshi for pointing out the issue).

v6 of this patch fixes more problems (thanks to Moritz Barsnick for
pointing them out).

v7 of this patch fixes use of sqrt() (changed to sqrtf(); thanks to
Moritz Barsnick for pointing this out). Also should be rebased off of
latest master branch commits at this point.

Signed-off-by: Stephen Seo <seo.disparate@gmail.com>