Improve per-key LED. Add extra example

- Small fixes to use all keys on GX502

aura/examples/pulser.rs

@@ -0,0 +1,36 @@
+use rog_aura::{AuraDbusWriter, GX502Layout, Key, KeyColourArray, KeyLayout};
+use std::ops::Sub;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let mut writer = AuraDbusWriter::new()?;
+
+    let mut key_colours = KeyColourArray::new();
+    let layout = GX502Layout::default();
+
+    writer.init_effect()?;
+    let rows = layout.get_rows();
+
+    let mut fade = 50;
+    let mut flip = false;
+    loop {
+        for row in rows {
+            for (k, key) in row.iter().enumerate() {
+                *key_colours.key(*key).1 = 255 / fade / (k + 1) as u8;
+            }
+        }
+
+        writer.write_colour_block(&key_colours)?;
+
+        if flip {
+            if fade > 1 {
+                fade -= 1;
+            } else {
+                flip = !flip;
+            }
+        } else if fade < 50 {
+            fade += 1;
+        } else {
+            flip = !flip;
+        }
+    }
+}