Add ext_qt patch for Windows to support fractional DPI scaling

This adds a series of patches related to QTBUG-53022 [1] for Windows to
allow enabling support for fractional DPI scaling.

These series of patches adds the option
QGuiApplication::setHighDpiScaleFactorRoundingPolicy to control how
the DPI scale factor is rounded (or not), which can also be overridden
by the environment variable QT_SCALE_FACTOR_ROUNDING_POLICY. The
options include:

• Round: Round up for .5 and above.
• Ceil: Always round up.
• Floor: Always round down.
• RoundPreferFloor: Round up for .75 and above.
• PassThrough: Don't round.

Currently the default option is explicitly set to Round to obtain the
old behaviour (where 1.5x is rounded to 2x).

Ref:

• https://bugreports.qt.io/browse/QTBUG-53022
• https://codereview.qt-project.org/157173
• https://codereview.qt-project.org/157174
• https://codereview.qt-project.org/161334
• https://codereview.qt-project.org/163219
• https://codereview.qt-project.org/176381
• https://codereview.qt-project.org/209886