Fix the scaling factor issue

[AsaMitakaIT]

The issue occurs when using the --force-scale-factor=<value> startup parameter with the gbm backend.

Before the modification, the logical size was incorrectly multiplied by the scaling factor, which caused the canvas to exceed the display.

I am not familiar with C++, so I’m unsure whether this has led to other issues.

Commands:

sudo apt install libegl1-mesa-dev
cmake -DBUILD_ELINUX_SO=ON -DBACKEND_TYPE=DRM-GBM -DCMAKE_BUILD_TYPE=<Debug || Rlease> ..
mv libflutter_elinux_gbm.so .../flutter-elinux/flutter/bin/cache/artifacts/engine/elinux-<ARCH>-<debug || release>/libflutter_elinux_gbm.so

[HidenoriMatsubayashi]

@taka-okayama could you watch the repos?