Use thrust counting iterator in mandelbrot1.cc

day4/examples/mandelbrot1.cc

-#include "CountingIterator.hh"
+#include <thrust/iterator/counting_iterator.h>
 #include <algorithm>
 #include <chrono>
 #include <complex>
@@ -8,6 +8,9 @@
 #include <string>
 #include <vector>
 
+template <class T>
+using CountingIterator = thrust::counting_iterator<T>;
+
 void save_pgm(std::string filename, size_t width, size_t height,
     std::vector<unsigned char> data)
 {
@@ -29,7 +32,7 @@ auto mandel(size_t width, size_t height) -> std::vector<unsigned char>
     double aspect = static_cast<double>(width) / height;
     std::vector<unsigned char> ans(width * height, 0);
     CountingIterator<size_t> beg { 0 }, end { width * height };
-    std::transform(std::execution::par, beg, end,
+    std::transform(std::execution::par_unseq, beg, end,
         ans.begin(), [=](size_t index) {
             double myrow = index / width;
             double mycol = index % width;