Restructure Pixel Rain interval code (#25516)

Co-authored-by: Joel Challis <git@zvecr.com>

quantum/rgb_matrix/animations/pixel_rain_anim.h

@@ -6,20 +6,27 @@ RGB_MATRIX_EFFECT(PIXEL_RAIN)
 #    ifdef RGB_MATRIX_CUSTOM_EFFECT_IMPLS
 
 static bool PIXEL_RAIN(effect_params_t* params) {
-    static fast_timer_t timer = 0;
-    static uint16_t     index = RGB_MATRIX_LED_COUNT + 1;
+    static uint8_t  index = 0;
+    static uint32_t timer = 0;
 
-    if ((params->iter == 0) && (timer_elapsed_fast(timer) > (320 - rgb_matrix_config.speed))) {
+    if (params->iter == 0 && params->init) {
         index = random8_max(RGB_MATRIX_LED_COUNT);
-        timer = timer_read_fast();
     }
 
     RGB_MATRIX_USE_LIMITS(led_min, led_max);
-    if (led_min <= index && index < led_max && HAS_ANY_FLAGS(g_led_config.flags[index], params->flags)) {
-        hsv_t hsv = (random8() & 2) ? (hsv_t){0, 0, 0} : (hsv_t){random8(), random8_min_max(127, 255), rgb_matrix_config.hsv.v};
-        rgb_t rgb = rgb_matrix_hsv_to_rgb(hsv);
-        rgb_matrix_set_color(index, rgb.r, rgb.g, rgb.b);
-        index = RGB_MATRIX_LED_COUNT + 1;
+    if (timer < g_rgb_timer) { // Execute when the delay period has elapsed
+        if (led_min <= index && index < led_max && HAS_ANY_FLAGS(g_led_config.flags[index], params->flags)) {
+            // Assign a random HSV color to hsv with 50% probability, otherwise assign zeroed hsv
+            hsv_t hsv = (random8() & 2) ? (hsv_t){0, 0, 0} : (hsv_t){random8(), random8_min_max(127, 255), rgb_matrix_config.hsv.v};
+            rgb_t rgb = rgb_matrix_hsv_to_rgb(hsv);
+            rgb_matrix_set_color(index, rgb.r, rgb.g, rgb.b);
+        }
+        if (!rgb_matrix_check_finished_leds(led_max)) {
+            // In the final LED range, update the LED index and advance the timer for
+            // the next cycle, scaling the delay between 256–2048 ms based on speed.
+            index = random8_max(RGB_MATRIX_LED_COUNT);
+            timer = g_rgb_timer + (2048 - scale16by8(1792, rgb_matrix_config.speed));
+        }
     }
     return rgb_matrix_check_finished_leds(led_max);
 }