FileDialogImplWin32: Use gdk_pixbuf_new_from_file_at_scale

...to load the preview GdkPixbuf.

Before this commit we loaded the GdkPixbuf at the original size and then downscale. This was inefficient and could hit the size limits of cairo surfaces.

Also backport a compilation fix from master: !6755 (merged) EDIT: merged in !7003 (merged)

Fixes inbox#11741