gl_video: handle non-mod-2 4:2:0 YUV video correctly

Allocate textures big enough to include the bottom/right borders (so the
chroma texture sizes are rounded up instead of down). Make the texture
large enough to include the additional luma border. Conceptually, we
pretend that the video frame is fully aligned, and then crop away the
unwanted borders. Filtering (even just bilinear) will access the
borders anyway, so it's possible that we might need to switch to
"harder" cropping instead, but at least pixels not close to the
border should be displayed correctly now.

Add a comment to mp_image.c about this luma border. These semantics are
kind of subtle, and the image allocation code handle this in a subtle
way too, so it's better to document this explicitly. The libavutil
image allocation code does similar things.
core: move contents to mpvcore (1/2)

core is used in many unix systems for core dumps. For that reason some tools
work under the assumption that the file is indeed a core dump (for example
autoconf does this).

This commit just renames the files. The following one will change all the
includes to fix compilation. This is done this way because git has a easier
time tracing file changes if there is a pure rename commit.
core: move contents to mpvcore (2/2)

Followup commit. Fixes all the files references.
input: add support for precise scroll axes

Support horizontal and vertical axes of input devices.

If the input device support precise scrolling with an input value then it
should first be scaled to a standard multiplier, where 1.0 is the default.

The multiplier will then applied to the following commands if possible:
 * MP_CMD_SEEK
 * MP_CMD_SPEED_MULT
 * MP_CMD_ADD

All other commands will triggered on every axis event, without change the
values specified in the config file.
wayland: add support for precise scrolling

The default value for a standard mouse is 10.0. Because we don't want to
multiply the value in the input config file we scale it down to 1.0.

Hopefully this should work for more precise mousewheels or touchpad, but
I don't have access to such hardware.
input.conf: better documentation and sane defaults

Uses the same defaults as BTN3/4/5/6 which are hardcoded by most backends for
the mouse wheel.
changes.rst: add precise scrolling support
demux_lavf: blacklist "tty" libavformat demuxer

This is completely useless, and in this particular case, it broke the
fallback for MLP2 subtitles (stored as .txt files) to demux_subreader.
(Yes, libavformat should be fixed to handle this, but for now this will
_always_ break playback of subtitle files stored in .txt.)

You can still force this demuxer, but by default we will just pretend
that the "tty" demuxer does not exist.