[Regression] The getVisibleElements helper function is broken in RTL-locales

[Snuffleupagus]

Attach (recommended) or Link to PDF file here: Not applicable

Configuration:

• Web browser and its version: All
• Operating system and its version: All
• PDF.js version: All versions since 2.0.943, including the current master.
• Is a browser extension: Not applicable

Steps to reproduce the problem:

1. Open http://localhost:8888/web/viewer.html#locale=he (or another RTL-locale)
2. Choose "Horizontal Scrolling"-mode in the Secondary Toolbar
3. Scroll to the end of the document.

What is the expected behavior? (add screenshot)
All pages, including the last one, should render successfully.

What went wrong? (add screenshot)
All pages, except the first ones, are completely blank.

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The bug is, as far as I can tell, caused by the getVisibleElements helper function not handling the viewer being used in RTL-locales (i.e. when dir="rtl" is set on the containing HTML-element).

While this very clearly effects the "Horizontal Scrolling"-mode, it's also possible that both the "Wrapped Scrolling"-mode and the "Odd/Even Spread"-modes are also affected (but to a lesser extent) in RLT-locales.

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I'd tentatively suggest that we label this as a regression, since it appears to have been broken in RTL-locales ever since PR #9208 which originally added the "Horizontal Scrolling"-mode (and others).