send preferred_buffer_scale event after entering an output

[lilydjwg]

Qt 6.7.0+ requires this to correctly scale when fractional scale is not enabled.

A simple PyQt6 program to demostrate:

#!/usr/bin/python3

import sys

from PyQt6 import QtWidgets, QtCore

def main():
  QtWidgets.QApplication.setHighDpiScaleFactorRoundingPolicy(
    QtCore.Qt.HighDpiScaleFactorRoundingPolicy.RoundPreferFloor)

  app = QtWidgets.QApplication(sys.argv)
  test = QtWidgets.QLabel("测试 Qt")
  test.show()
  sys.exit(app.exec())

if __name__ == '__main__':
  main()

Setting QT_SCALE_FACTOR_ROUNDING_POLICY=RoundPreferFloor for Qt6 apps should also reproduce it but not tested. This happens with Telegram (with "presice High DPI scaling" disabled) + Qt6 6.7.0.

Fixes #2253

[ammen99]

In general maybe we should iterate over all the visible outputs and select the highest scale?

[lilydjwg]

We may select the highest one among the outputs the surface in on.

[ammen99]

LGTM now, thanks!