Retain brightness with lighting layers (qmk#13025)

Add guard `RGBLIGHT_LAYERS_RETAIN_VAL` to retain the currently used val
when applying lighting layers.

Author: pstadler

docs/feature_rgblight.md

@@ -326,6 +326,10 @@ would turn the layer 0 (or 1) on and off again three times when `DEBUG` is press
 
 Normally lighting layers are not shown when RGB Lighting is disabled (e.g. with `RGB_TOG` keycode). If you would like lighting layers to work even when the RGB Lighting is otherwise off, add `#define RGBLIGHT_LAYERS_OVERRIDE_RGB_OFF` to your `config.h`.
 
+### Retain brightness
+
+Usually lighting layers apply their configured brightness once activated. If you would like lighting layers to retain the currently used brightness (as returned by `rgblight_get_val()`), add `#define RGBLIGHT_LAYERS_RETAIN_VAL` to your `config.h`.
+
 ## Functions
 
 If you need to change your RGB lighting in code, for example in a macro to change the color whenever you switch layers, QMK provides a set of functions to assist you. See [`rgblight.h`](https://github.com/qmk/qmk_firmware/blob/master/quantum/rgblight/rgblight.h) for the full list, but the most commonly used functions include:

quantum/rgblight/rgblight.c

@@ -694,6 +694,9 @@ bool rgblight_get_layer_state(uint8_t layer) {
 
 // Write any enabled LED layers into the buffer
 static void rgblight_layers_write(void) {
+#    ifdef RGBLIGHT_LAYERS_RETAIN_VAL
+    uint8_t current_val = rgblight_get_val();
+#    endif
     uint8_t i = 0;
     // For each layer
     for (const rgblight_segment_t *const *layer_ptr = rgblight_layers; i < RGBLIGHT_MAX_LAYERS; layer_ptr++, i++) {
@@ -714,7 +717,11 @@ static void rgblight_layers_write(void) {
             // Write segment.count LEDs
             LED_TYPE *const limit = &led[MIN(segment.index + segment.count, RGBLED_NUM)];
             for (LED_TYPE *led_ptr = &led[segment.index]; led_ptr < limit; led_ptr++) {
+#    ifdef RGBLIGHT_LAYERS_RETAIN_VAL
+                sethsv(segment.hue, segment.sat, current_val, led_ptr);
+#    else
                 sethsv(segment.hue, segment.sat, segment.val, led_ptr);
+#    endif
             }
             segment_ptr++;
         }