Fix disableAutoFetch regression in the generic viewer

[Snuffleupagus]

After PR #5263, setting disableAutoFetch = true no longer works correctly (the entire file loads). This is a tentative patch that attempts to address that.

[yurydelendik]

hmm, that's not really a regression by design (to enable incomplete file loading both disableStream=true and disableAutoFetch=true must be set), since enabling stream shall automatically disableAutoFetch. we have a confusion here with the options and this PR might be an acceptable solution. however it's better to see something like enumeration autoFetch=[disabled|stream|range|auto]:

• disable: disableAutoFetch=true, disableStream=true;
• stream: disableAutoFetch=true, disableStream=false;
• range: disableAutoFetch=false, disableStream=true;
• auto: disableAutoFetch=false, disableStream=false; (if stream is not available, falls to auto-fetch)

[yurydelendik]

Just thinking: if we let disableAutoFetch to disable stream, there is really no need in disableStream

[Snuffleupagus]

hmm, that's not really a regression by design (to enable incomplete file loading both disableStream=true and disableAutoFetch=true must be set),

A valid point, that I completely overlooked, sorry about that!
This works well in the addon given the code in PdfStreamConverter.jsm, but not in the generic viewer (where it is a regression, in my opinion).
@yurydelendik Any objection against me re-purposing this PR to just fix the generic viewer, and then we can explore your enumeration idea in a followup if we want to?

[yurydelendik]

I see. Yeah we need to fix "disableStream=true and disableAutoFetch=true" thing for the generic viewer

[yurydelendik]

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