Make some examples deterministic

[mockersf]

Objective

• Improve reproducibility of examples

Solution

• Use seeded rng when needed
• Use fixed z-ordering when needed

Testing

steps=5;
echo "cpu_draw\nparallel_query\nanimated_fox\ntransparency_2d" > test
cargo run -p example-showcase -- run --stop-frame 250 --screenshot-frame 100 --fixed-frame-time 0.05 --example-list test --in-ci;
mv screenshots base;
for prefix in `seq 0 $steps`;
do
  echo step $prefix;
  cargo run -p example-showcase -- run --stop-frame 250 --screenshot-frame 100 --fixed-frame-time 0.05 --example-list test;
  mv screenshots $prefix-screenshots;
done;
mv base screenshots
for prefix in `seq 0 $steps`;
do
  echo check $prefix
  for file in screenshots/*/*;
  do
    echo $file;
    diff $file $prefix-$file;
  done;
done;