X11: set fallback logical DPI to 96

X11: set fallback logical DPI to 96

Origin: upstream, https://code.qt.io/cgit/qt/qtbase.git/commit?id=7238123521708ec9
Last-Update: 2025-12-31

Returning physical DPI from logicalDpi() is problematic,
as explained in commit 77e04acb.

The most predictable implementation is to never return
physical DPI from QPlaformScreen::logicalDpi(). Other
platform plugins already do this, and this change
brings xcb in line with the rest of Qt.

We have the QPlatformScreen::physicalSize() API which
covers returning physical DPI (indirectly); Options
for selecting which one to use can be implemented on
top of these (see QT_USE_PHYSICAL_DPI).

Gbp-Pq: Name dont_use_physical_dpi.diff