app: fix #6961 Scaled image bug

With large image sizes a 32-bit int is not enough for the intermediate
computations, which byte per pixel, width and height are.

So, just like the function below it does: gimp_gegl_pyramid_get_memsize,
we will cast these to gint64.

Thanks to Massimo Valentini for finding the cause.
2 jobs for master in 73 minutes and 39 seconds (queued for 3 seconds)
Name Stage Failure
flatpak Gimp
Uploading artifacts...
flatpak-builder.log: found 1 matching files and directories

WARNING: gimp-git.flatpak: no matching files
WARNING: repo.tar: no matching files
WARNING: .flatpak-builder/build/gimp/_flatpak_build/meson-logs/meson-log.txt: no matching files
WARNING: .flatpak-builder/build/gimp/_flatpak_build/meson-logs/testlog.txt: no matching files
Uploading artifacts as "archive" to coordinator... 201 Created
id=2029873 responseStatus=201 Created token=K2aM6yPM
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1