pcm/Export: add GetInputFrameSize()

src/pcm/Export.cxx

@@ -141,7 +141,7 @@ PcmExport::GetOutputFrameSize() const noexcept
 	}
 #endif
 
-	return channels * sample_format_size(src_sample_format);
+	return GetInputFrameSize();
 }
 
 unsigned

src/pcm/Export.hxx

@@ -184,6 +184,14 @@ public:
 	 */
 	void Reset() noexcept;
 
+	/**
+	 * Calculate the size of one input frame.
+	 */
+	gcc_pure
+	size_t GetInputFrameSize() const noexcept {
+		return channels * sample_format_size(src_sample_format);
+	}
+
 	/**
 	 * Calculate the size of one output frame.
 	 */