This allows the compiler to discard buffer contents between two
function calls.

Same bug as in the previous commit.

When the data chunk size is not a multiple of the frame size, the last
partial frame lead to an endless loop.  We fix this by checking
chunk_sze>=frame instead of chunk_sze>0.  This way, the partial frame
is simply skipped.

Reduce header dependencies.

Seeking on NFS or SMB is cheap.  Actually, only HTTP streams are
expensive to seek.  This enables a few features on NFS/SMB files, for
example Ogg tags.

Has been moved to class InputStream long ago.

Since we already checked InputStream::KnownSize(), we can assume that
GetSize() returns a valid value, and this check is obsolete.

Replace the bogus GetSize() check and call GetSize() only when
necessary.

Variable-length arrays are not allowed in C++.

If the offset is out of range, the seek will simply fail.  Not a
problem.

The plugin assumes that the size is known, but does not verify it at
runtime.

Generate more detailed error messages.

Eliminate duplicate definition (in input plugin and decoder plugin).