check sixel hls to rgb colour conversion
Xterm commit 000c58558258e8d10638bc2866bd54062a613e8d (version xterm-384g) changes its graphics.c:hls2rgb()
function with the changelog saying that these are fixes for ReGIS, to "correct vonversion from HLS to RGB" (the function is also used for sixel in xterm). Since vte's sixel-context.cc:Context::make_color_hls()
is derived from, and tested against, this xterm code (see sixel-test.cc
), we need to check what that xterm change is doing, if it's correct, and if so, fix our implementation.