Fix progression of parser

[sgilson]

In the current version, the calculation for how far the parser should progress through the content has a bug. In larger documents (and for some reason when using SVGs in particular), the parser will skip over large chunks of the content and leave diagrams un-rendered.

I can't share the source document and I haven't yet been able to create a reasonably sized test case to reproduce the issue. However, I've taken some small steps to confirm the fix:

• With current version: Printed the text at the location of idx after it is updated. Confirmed suspicion that the pointer is arbitrarily jumping around in the document
• With fix: Same approach as above. Pointer now appears to progress to the point right after the parsed text as expected
• Tested on document containing the problem. Code blocks that were previously skipped are now rendered
• Verified existing tests pass

Let me know if I am missing something, but I think this fix is fairly straightforward.

[mikitex70]

Hi @sgilson, thanks for your Pull Request. It's now merge in the master as commit 845580e. I've not checked it with big documents, but I'm sure its working fine. Thanks again.