First page on it's own spread not working in RTL mode

**Description**

This can be reproduced as follows: on the "bookreader simple demo" compare the demo as is, with what happens when you add the following line to the `options` object in `BookReaderJSSimple.js` :
`pageProgression: 'rl',`
(switching in each case to two-page mode)
Then, instead of the first page being on it's own spread, it's combined with the next page.

**Evidence / Screenshot (if possible)**
LTR mode:

RTL mode:

**Expectation**

Since each value in the `data` array is supposed to represent a "spread", and this first item contains only the object for the first page in both cases, I would except the first page to be on it's own spread whatever the setting of `pageProgression`.

**Context**

Windows 10, Chrome 120.0.6099.130, BookReader 5.0.0-77