x11_common: minor simplification
bstr: add bstrspn()
bstr: add bstrto0()
m_option: split out sub-config parsing
options: untangle track range parsing for stream_cdda

Remove the "object settings" based track range parsing (needed by
stream_cdda only), and make stream_cdda use CONF_TYPE_INT_PAIR.

This makes the -vf parsing code completely independent from other
options. A bit of that code was used by the mechanism removed with
this commit.
m_option: add function to check whether parameters are required

To avoid that it will be duplicated with m_option.c and m_config.c.
m_option: redo code for parsing -vf to accept quotes

Parsing sub-configs (like --rawvideo=subopts or the suboptions for
--vo=opengl:subopts) was completely different from the -vf parsing code
for a variety of reasons. This change at least makes -vf use the same
splitter code as sub-config options.

The main improvement is that -vf now accepts quotes, so you can write
things like:

     -vf 'lavfi=graph="gradfun=10:20"'

(The '' quotes are for shell escaping.)

This is a rather big and intrusive change. Trying some -vf lines from
etc/encoding-example-profiles.conf seems to confirm it still works.

This also attempts to unify one subtle difference in handling of
positional arguments. One consequence is that a minor detail changes.
Sub-configs don't know positional arguments, and something like "--
opt=sub1=val1:sub2" means that sub2 has to be a flag option. In -vf
parsing, sub2 would be a positional option value. To remove this
conflict and to facilitate actual unification of the parsers in the
future, the sub2 will be considered a flag option if and only if such a
flag option exists. Otherwise, it's considered a value for a positional
option.

E.g. if there's a filter "foo" with a string option "sopt" and a flag
option "fopt", the behavior of the following changes:

   -vf foo=fopt

Before this commit, this would set "sopt=fopt" in the filter. Now, it
enables the fopt flag, and the sopt option remains unset. This is not an
actual problem to my knowledge.
mp_image: provide function to convert mp_image to AVFrame

Note that this does not pass through QP information (qscale field).
The only filter for which this matters is vf_pp, and we have this
natively.
vf_lavfi: add libavfilter bridge

Requires recent FFmpeg/Libav git versions. Earlier versions will not
be supported, as the API is different. (A libavfilter version that
uses AVFrame instead of AVFilterBuffer is needed.)

Note that this is sort of useless, because the option parser prevents
you from making use of the full libavfilter graph syntax. This has to be
fixed later.

Most of the filter creation code (half of the config() function) has
been taken from avplay.c.

This code is not based on MPlayer's vf_lavfi. The MPlayer code doesn't
compile as it hasn't been updated through multiple libavfilter API
changes, making it completely useless as a starting point.