Fix GIFV encoding params (#7098)

- Explicitly specify video codec.
  When ffmpeg isn't compiled with libx264 but openh264, mpeg4 is selected as video codec.
- Swap avarage bitrate and max bitrate.

app/models/media_attachment.rb

@@ -130,8 +130,9 @@ class MediaAttachment < ApplicationRecord
                 'pix_fmt'  => 'yuv420p',
                 'vf'       => 'scale=\'trunc(iw/2)*2:trunc(ih/2)*2\'',
                 'vsync'    => 'cfr',
-                'b:v'      => '1300K',
+                'c:v'      => 'h264',
-                'maxrate'  => '500K',
+                'b:v'      => '500K',
+                'maxrate'  => '1300K',
                 'bufsize'  => '1300K',
                 'crf'      => 18,
               },