Fix #308562: 1-line staves show unexpected vertical offset

[elerouxx]

...on note entry mode.

Resolves: https://musescore.org/en/node/308562

The note entry cursor is drawn about 2 lines below the actual mouse pointer in 3.5 RC, when hovering over staves that are "born" as 1-line (even if a Staff Type Change sets it to 5-lines later as in second image below).

This is a side effect of an yOff added to 1-line system staves in order to fix issues with brackets.
I just added a complementary code to getPosition() to fix this side effect.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• [N/A] I created the test (mtest, vtest, script test) to verify the changes I made

[Jojo-Schmitz]

the AppVeyor fail should be fixed by a rebase

[elerouxx]

the AppVeyor fail should be fixed by a rebase

Done!