Thumbnails appear pixelated

[libellula]

Hi Clsid2,
first of all, thanks for continuing the development of this great player.
I just wanted to report something I noticed which seems odd since MPC-BE, with the same setting, does it well.
I posted two thumbnails of the same file, with the same settings, one made by MPC-HC, the other made by MPC-BE. I chose a sample video which exposes the issue very well. My OS is Windows 10 Pro x64, the video renderer is MadVR, but even wth other renderers the issue stays there.

[clsid2]

Give a link to that video file.

[libellula]

It happens to every video, this one just exposes it more. BTW,

fish_-_62123.Original.mp4

[clsid2]

Seems to be due to scaling filter used:
https://github.com/Aleksoid1978/MPC-BE/blob/f9f8d22f89ed27cf3eb22d78c37885237c60f573/src/apps/mplayerc/ThumbsTaskDlg.cpp#L218

[v0lt]

To get the correct result when the image is greatly reduced, you need to use convolution.
Interpolation algorithms will perform poorly for reduction. This is especially noticeable in video frames with sharp edges.

[adipose]

#2802

Similar method as mpc-be, but I used stbir_resize as we already have it in the project.