sd_lavc: remove ineffective code

It makes no sense to set the packet duration, because libavcodec doesn't
know the timebase. And in fact, no subtitle decoder accesses the packet
duration, except text subtitle converters, which are not relevant here.
So this code did nothing - drop it.

Also fix a blatantly incorrect comment.
demux: avoid unnecessary wakeups

If a packet is appended to a stream, and there were already packets
queued, nothing about the state changed, as far as the user (i.e. the
player) is concerned. Thus no wakeup is needed.

The pthread_cond_signal() call following this is not interesting - it
will simply be a NOP if there are actually no waiters.
win32: correct HANDLE type

The correct type is HANDLE, not HANDLE*, though this change shouldn't
affect functionality.
win32: correct SGR sequence handling

This should get colour working again on the Windows console.

Fixes #1032.
vo_opengl: add spline64 filter kernel

The coefficients was taken from fmtconv plugin for vapoursynth:
https://github.com/vapoursynth/fmtconv/blob/master/src/fmtc/ContFirSpline64.cpp

The package is licensed under WTFPL, and it's from the same author
of Dither plugin for avisynth.
vo_opengl: add taps setting for two filter kernels

Added taps settings for sinc and lanczos kernel. Kernel radius can be
set manually by user now. Currently mpv supports kernel radius up to 8
(same as taps=16).

Name of the variable 'radius' was changed in order to prevent
unexpected access to this variable: it's only supposed to be used in
filter initialization.
vo_opengl: add cparam1 and cparam2 options

Although cscale is rarely used, it's possible that params of cscale
are accidentally set to lparam1 and lparam2, which might cause
unexpected outcomes.
vo_opengl: Add parameter to gaussian filter

Added a new parameter 'p' to gaussian filter. The new formula used
a different base taken from fmtconv plugin, so that the
new parameter is exactly same as the one used in Avisynth and
Vapoursynth.

The new default value is 2 / log(2) * 10, with the default value it
conforms to the original kernel taken from vector-agg.