view: don't try to restore to very small width/height on unmaximize

view: don't try to restore to very small width/height on unmaximize

Thonny (Python IDE made with Tk) may set the window geometry to 1x1 and
maximizes the window before mapping. This set `view->natural_geometry`
to 1x1, so labwc tried to restore the window geometry to it on
unmaximize, causing validation errors in `ssd_update_geometry()` as its
width and height are smaller than `LAB_MIN_VIEW_{WIDTH,HEIGHT}`.

This commit fixes it by not allowing geometries smaller than
`LAB_MIN_VIEW_{WIDTH,HEIGHT}` in `view->natural_geometry`.