Fix severe perf degradation reading Stored file in Mono

When reading a stored file, ZipFile returns a PartialInputStream
directly instead of wrapping it with a decompression layer.

PartialInputStream handles sharing the underlying stream among
multiple threads by calling Seek() before each read.

Unlike Microsoft's reference implementation, Mono's FileStream
flushes its internal read buffer on every Seek.

Author: dubois

src/Zip/ZipFile.cs

@@ -3778,8 +3778,11 @@ public override int Read(byte[] buffer, int offset, int count)
 							return 0;
 						}
 					}
-					
-					baseStream_.Seek(readPos_, SeekOrigin.Begin);
+					// Protect against Stream implementations that throw away their buffer on every Seek
+					// (for example, Mono FileStream)
+					if (baseStream_.Position != readPos_) {
+						baseStream_.Seek(readPos_, SeekOrigin.Begin);
+					}
 					int readCount = baseStream_.Read(buffer, offset, count);
 					if (readCount > 0) {
 						readPos_ += readCount;