BAdapterIO: Refine size inheritance

* Wait evaluation is done considering both position
and buffer size.
* Set back position to 0 after seeking.
* Simplify EvaluatePosition to take advantage of inheritance.
* Use back position to determine the current size of the buffer.
* Add more debug tracing.
* Other minor fixes.

headers/private/media/AdapterIO.h

@@ -70,7 +70,7 @@ protected:
 			ssize_t					BackWrite(const void* buffer, size_t size);
 
 private:
-			status_t				_EvaluateWait(off_t position);
+			status_t				_EvaluateWait(off_t position, off_t size);
 
 			int32					fFlags;