Annotations: problem with creation of invisible annotations

 * Xreader version: 2.8.3
 * Distribution - (Mint 20.1 Cinnamon 4.8.6

Issue
It is possible to create an annotation that looks good to go, but which becomes invisible when the notelet is closed.

Steps to reproduce

1. Open the attached document.
   test_doc.pdf

2. Navigate so that page 2 is indicated in the "Select Page" field, but allow the end of page 1 to also be at the top of the viewport.

3. Add a text annotation to the text in Page 1. Label it "1", say. Notice that though the notelet is open, there is no annotation icon. Close the notelet.

Actual behaviour: The annotation reference shows up in the sidebar but is (understandably) not visible on the page. When you highlight the reference, a dotted outline of the annotation is visible in its original location but you cannot do anything with it.

Expected behaviour

Xreader should only allow annotations to be applied on the page indicated in the "Select Page" field.

Start by reproducing the issue with test_doc.pdf, following the viewport and Select Page steps while creating a text annotation. Trace annotation creation and page selection handling; done means annotations are restricted to the page shown in Select Page and remain visible after the notelet closes.