output/alsa: improve silence generator test in DrainInternal()

There cannot be partial frames in the buffer, so we don't need
GetPeriodPosition(); it's enough to check whether head has been moved
in this period.

src/lib/alsa/PeriodBuffer.hxx

@@ -73,6 +73,14 @@ public:
 		return tail >= capacity;
 	}
 
+	/**
+	 * Has some data already been consumed at the buffer's head in
+	 * this period?
+	 */
+	bool WasConsumed() const noexcept {
+		return head > 0;
+	}
+
 	/**
 	 * Returns the tail of the buffer, i.e. where new data can be
 	 * written.  Call GetSpaceBytes() to find out how much may be
@@ -163,10 +171,6 @@ public:
 		ConsumeBytes(n * frame_size);
 	}
 
-	snd_pcm_uframes_t GetPeriodPosition(size_t frame_size) const noexcept {
-		return head / frame_size;
-	}
-
 	void Rewind() noexcept {
 		head = 0;
 	}