gdk: Revert making get_size optional

gdk: Revert making get_size optional

49a0ec6e95408e3484397602ad46e274cedbf98b added this hunk without
explanations and it's obviously wrong because failing to have a get_size
vfunc will now not set width and height and they'll end up with invalid
values.

In fact, the GdkDrawableImpl{X11,Win32,Quartz} functions do not have
get_size implemented, so the patch was most likely an attempt to work
around calling this function erroneously on the impl objects.

The correct fix is to call impl->wrapper instead.