Use ffmpeg for thumbnailer

Finally, we compile the results using the best workers/threads
combination for each option from 02b6f6e and c403b41:

| mpv/ffmpeg   |          Samples         |
| ------------ | H.264 | HEVC | VP9 | AV1 |
| mpv (8/2)    |   10s |  29s | 57s | 16s |
| ffmpeg (4/2) |   10s |  29s | 47s | 71s |

However, I'm not convinced with the results as there were two outliers.
So, I decided to test another set, this time focusing only on VP9 and
AV1 videos.

| mpv/ffmpeg   |        Samples        |
| ------------ | VP9_1 | AV1_1 | AV1_2 |
| mpv (8/2)    |   35s |   49s |  127s |
| ffmpeg (4/2) |   28s |   41s |  108s |

That's more like it; ffmpeg is now consistently equal to or faster than
mpv in all cases. I wonder which of my encoding settings skyrocketed mpv
but crippled ffmpeg...

script-opts/thumbnailer.conf

@@ -29,13 +29,13 @@ bitrate_delta_factor=1
 #update_time=0.25
 
 # Worker
-max_workers=8
+max_workers=4
 worker_remote_factor=1
 #worker_bitrate_factor=1
 #worker_delay=1
 #worker_timeout=5
 #accurate_seek=yes
-#use_ffmpeg=yes
+use_ffmpeg=yes
 #prefer_ffmpeg=yes
 ffmpeg_threads=2
 #ffmpeg_scaler=spline

scripts/Thumbnailer.lua

@@ -295,7 +295,7 @@ local function worker_set_options()
         ffmpeg_threads        = user_opts.ffmpeg_threads,
         ffmpeg_scaler         = user_opts.ffmpeg_scaler,
         ffmpeg_hwaccel        = user_opts.ffmpeg_hwaccel,
-		ffmpeg_hwaccel_device = user_opts.ffmpeg_hwaccel_device
+        ffmpeg_hwaccel_device = user_opts.ffmpeg_hwaccel_device
     }
 end