Fix exception during vertical paragraph navigation (nvaccess#16276)

Closes nvaccess#16274

Summary of the issue:
Exception during vertical navigation when one of page items is offscreen.

Description of user facing changes
N/A

Description of development approach
Previously I was retrieving x coordinate by calling textInfo.pointAtStart. However it appears that this doesn't work when said textInfo is off screen.
Switching to another method:
textInfo.NVDAObjectAtStart.location[0]

source/browseMode.py

@@ -503,7 +503,7 @@ def iterFactory(direction: str, pos: textInfos.TextInfo) -> Generator[TextInfoQu
 		elif itemType == "verticalParagraph":
 			def paragraphFunc(info: textInfos.TextInfo) -> int | None:
 				try:
-					return info.pointAtStart.x
+					return info.NVDAObjectAtStart.location[0]
 				except AttributeError:
 					return None