Enable 4:2:0 chroma subsampling for JPEG thumbnails

Enable 4:2:0 chroma subsampling for JPEG thumbnails

* Add $wgJpegPixelFormat, default to 'yuv420'
* Implemented for ImageMagick via CLI and extension
* Currently ignored for other scaler backends
* Added test case to run when using ImageMagick

4:2:0 subsampling can save an average of 17% bandwidth
over 4:4:4 subsampling, at the cost of some artifacting
at sharp red or blue edges. This is usually not noticeable
in photographic images.

To restore the previous behavior, set to false:

  $wgJpegPixelFormat = false;

which will maintain the original file's pixel subsampling
settings in the thumbnail.

Can set explicitly to one of:

  'yuv444' - never subsample
  'yuv422' - subsample 2x horizontally, not vert
  'yuv420' - subsample 2x in both dimensions

Bug: T129128
Change-Id: Ib9cb36c3a7e6a69d66c11150ef4a1d02dbac2df5