TV: Ummute sound if volume is adjusted.

This avoids potential confusion if the sound is muted and changing the
volume then does nothing - leading the user to think there is a problem
with the sound. An entirely un-scientific survey of 3 TVs and a couple
of STBs seems to suggest this is more or less standard behaviour.

mythtv/libs/libmythtv/tv_play.cpp

@@ -7776,6 +7776,9 @@ void TV::ChangeVolume(PlayerContext *ctx, bool up)
         return;
     }
 
+    if (ctx->player->IsMuted())
+        ToggleMute(ctx);
+
     uint curvol = ctx->player->AdjustVolume((up) ? +2 : -2);
     ctx->UnlockDeletePlayer(__FILE__, __LINE__);