subtitles: change default libass rendering style

subtitles: change default libass rendering style

Change the default style used for rendering plaintext subtitles with
libass. The following attributes change:

* FontSize: Increase the size multiplier used with the default
  -subfont-autoscale 3 from 1.4 to 1.7, thus making the font 21%
  larger. Actually implementing the -subfont-autoscale modes other
  than 1 would give a multiplier of 5/3=1.67 for 4:3 aspect ratio
  video, sqrt(337)/9=2.04 for 16:9.

* PrimaryColour: Change color from yellow to white.

* Outline: Before this was 2 regardless of user font size choice. Make
  it FontSize/16 instead. With default font size this is about half
  the previous width.

* Kerning: Enable kerning for the track. There won't be styling
  designed for VSFilter's lack of kerning, and hopefully people won't
  use broken fonts with bad kerning tables for the default font
  either.