FIX (EXTREMELY TRICKY) BUG concerning reusing TeXBlockSpan
MUST compare cached_line_spans[b] with TeXBlockSpan.DUMMY_TEX_BLOCK (instead of null) to decide whether we can simply update its data or must reuse/create new object!
Add more DEBUG_* switches and debuging code for document
Fill reusable span array with TeXBlockSpan.DUMMY_TEX_BLOCK at construction time