Make jxl quality behave more like jpg quality

Summary:
Experimentation has shown that for a particular quality setting Q, jxl compresses "harder", meaing the resulting file is much smaller, but also, the MS-SSIM score is much lower, making it difficult to compare behavior and switch between jpg and jxl.
This diff "bumps" the effective jxl quality, so as make jxl generate files with a slightly higher score than jpg for quality settings between 80 and 95, while remaining smaller than jpg files. Note that this behavior is very data dependent and the observation made while using rgb8 images. There is no ambition that this adjustment is perfect, only to make behaviors much more coherent. Note that H.265 and H.264 generate even better MS-SSIM scores for this range.
Without this change, jxl is just way out of the pack.

Reviewed By: kiminoue7

Differential Revision: D51570593

fbshipit-source-id: 7a3b751101c22d796a0fd27d79b9748842278f0a

vrs/utils/PixelFrameJxl.cpp

@@ -100,6 +100,9 @@ JxlEncoder* getThreadJxlEncoder() {
 }
 
 inline float percent_to_butteraugli_distance(float quality) {
+  // Make quality behave more like jpg's quality setting accoding to MS-SSIM. Very empirical.
+  float to100 = 100 - quality;
+  quality = 100 - to100 / 4;
   // Quality calculation inspired by cjxl.cc
   // Extended to work meaningfully between 99.99 and 99.999, so with quality = 99.999,
   // the file size is now getting close to that of lossless.