make merge_heat_counter more efficientHEADmaster

Accessing the underlying buffers directly and iterating over them is
more efficient (and gives the compiler more options to optimize) than
the dance with get_pixel_mut.

On my local tests with 119 tracks, this shaves off ~30 seconds (from 90
seconds down to 60 seconds).

1 files changed, 2 insertions, 3 deletions

diff --git a/hittekaart/src/renderer/heatmap.rs b/hittekaart/src/renderer/heatmap.rs
index 187c8f3..38a1543 100644
--- a/hittekaart/src/renderer/heatmap.rs
+++ b/hittekaart/src/renderer/heatmap.rs
@@ -118,9 +118,8 @@ fn render_line<P: Pixel>(
 fn merge_heat_counter(base: &mut HeatCounter, overlay: &HeatCounter) {
     for (tx, ty, source) in overlay.enumerate_tiles() {
         let target = base.tile_mut(tx, ty);
-        for (x, y, source) in source.enumerate_pixels() {
-            let target = target.get_pixel_mut(x, y);
-            target[0] += source[0];
+        for (t, s) in target.iter_mut().zip(source.iter()) {
+           *t += *s;
         }
     }
 }