PackageFileHeapWriter: Fix update case

* Pull _UnwriteLastPartialChunk() out of Reinit() for reuse.
* _UnwriteLastPartialChunk(): fPendingDataSize wasn't set.
* _PushChunks(): Some simplifications for clarity.
* ChunkBuffer/RemoveDataRanges(): Use data reading and decompression
  methods provided by our base class instead of duplicating the
  implementation.
* RemoveDataRanges():
  - _FlushPendingData() before starting, so we don't ignore the pending
    data and _UnwriteLastPartialChunk() when done, so a partial chunk
    is read back into the pending data buffer.
  - fUncompressedHeapSize wasn't reset before the main processing loop,
    thus resulting in an erroneous size later on.

headers/private/package/hpkg/PackageFileHeapWriter.h

@@ -68,6 +68,8 @@ private:
 			struct ChunkSegment;
 			struct ChunkBuffer;
 
+			friend struct ChunkBuffer;
+
 private:
 			void				_Uninit();
 
@@ -81,6 +83,7 @@ private:
 
 			void				_PushChunks(ChunkBuffer& chunkBuffer,
 									uint64 startOffset, uint64 endOffset);
+			void				_UnwriteLastPartialChunk();
 
 private:
 			void*				fPendingDataBuffer;