webmsplit: Support high data rate files.

The WebM splitter DShow filter has been unable to support
high data rate files due to only being able to allocate
64K of memory at a time. Also modifies
mkvparser::Stream::SetCurr to pass back errors to callers
when they occur.

Bug: https://code.google.com/p/webm/issues/detail?id=651
Change-Id: I30fefd53c31289a0db7a63c24de07cb48e2055bb