Set global howler volume, use log volume scale internally

fixes:
- next track volume
- saves precise volume slider position
- keyboard shortcut now sets volume in correct scale

changes/changelog.d/1542.bugfix

@@ -0,0 +1 @@
+Use global Howler volume instead of setting it separatly for each track (fixes #1542)

changes/changelog.d/1543.bugfix

@@ -0,0 +1 @@
+Store volume in logarithmic scale and convert when setting it to audio (fixes #1543)

front/src/components/audio/Player.vue

@@ -226,6 +226,7 @@
 <script>
 import { mapState, mapGetters, mapActions } from "vuex"
 import GlobalEvents from "@/components/utils/global-events"
+import { toLinearVolumeScale } from '@/audio/volume'
 import { Howl } from "howler"
 import $ from 'jquery'
 import _ from '@/lodash'
@@ -265,8 +266,6 @@ export default {
     this.$store.commit("player/isLoadingAudio", false)
     Howler.unload()  // clear existing cache, if any
     this.nextTrackPreloaded = false
-    // we trigger the watcher explicitely it does not work otherwise
-    this.sliderVolume = this.volume
     // this is needed to unlock audio playing under some browsers,
     // cf https://github.com/goldfire/howler.js#mobilechrome-playback
     // but we never actually load those audio files
@@ -364,7 +363,6 @@ export default {
         loop: false,
         html5: true,
         preload: true,
-        volume: this.volume,
         onend: function () {
           self.ended()
         },
@@ -757,11 +755,12 @@ export default {
       },
       immediate: false
     },
-    volume(newValue) {
-      this.sliderVolume = newValue
-      if (this.currentSound) {
-        this.currentSound.volume(newValue)
-      }
+    volume: {
+      immediate: true,
+      handler(newValue) {
+        this.sliderVolume = newValue
+        Howler.volume(toLinearVolumeScale(newValue))
+      },
     },
     sliderVolume(newValue) {
       this.$store.commit("player/volume", newValue)

front/src/components/audio/VolumeControl.vue

@@ -38,7 +38,6 @@
 </template>
 <script>
 import { mapState, mapGetters, mapActions } from "vuex"
-import { toLinearVolumeScale, toLogarithmicVolumeScale } from '@/audio/volume'
 
 export default {
   data () {
@@ -50,10 +49,10 @@ export default {
   computed: {
     sliderVolume: {
       get () {
-        return toLogarithmicVolumeScale(this.$store.state.player.volume)
+        return this.$store.state.player.volume
       },
       set (v) {
-        this.$store.commit("player/volume", toLinearVolumeScale(v))
+        this.$store.commit("player/volume", v)
       }
     },
     labels () {