Mermaid's xychart-beta cannot draw grouped bars or a legend: two bar
series overlap on the same axis position, so the paired GPU-vs-cairo
comparison was unreadable. Render the charts as images with real
grouped bars, value labels and a legend (bench/make-charts.py), and
embed them from .github/assets/.
The README already pointed to run-bench.sh and run-bench-hires.sh as the
way to reproduce the performance tables; add them under bench/ along with
gl-bench.el (the five workloads) and a README describing the methodology.
The scripts are relocatable: the binary is taken from $EMACS, gl-bench.el
is found relative to the script, and results go to a temporary directory.
