renderer: use fg as extension color for covering glyphs (U+2588)

[mitchellh]

Fixes #2099

This is another heuristic of sorts to make window-padding-color=extend look better by default. If a fully covering glyph is used then we use the fg color to extend rather than the background.

This doesn't account for fonts that may do this for whatever codepoints, but I think that's a special scenario that we should just recommend disabling this feature.

[mitchellh]

Seeing some issues with Metal still, I suspect some Metal shader issue but not sure yet.

The shell script below reproduces. On my system it produces the following image, all red blocks should be extended. They are on OpenGL:

#!/usr/bin/env bash

function csi() {
  printf "\033[%s" "$*"
}

function osc() {
  printf "\033]%s\007" "$*"
}

rows=$(tput lines)
cols=$(tput cols)

csi "2J"
csi "1;1H"
csi "44m" # blue bg
csi "31m" # red fg
for ((j=1; j<=cols; j++)); do
  printf " "
done
csi "1;1H"

printf "\u2588"
printf "1"
printf "\u2588"
printf "fish"
printf "\u2588"
csi "0m"
printf "\n"
read