Accept line height scaling factor less than 1.0
vte has supported increasing line height since 26c79c6d. It will clamp the cell height scaling factor above
VTE_CELL_SCALE_MIN, which is 1.0 currently, so users can't make lines narrower than what the font specifies.
This may sound like a niche feature, however it does have some use cases: vte updated the way it calculates the inistric line height of a font in #163 (closed). Some user dislike the new behavior, since the line height increases and vertical space is wasted. Relaxing the check on
VTE_CELL_SCALE_MIN will give users the option to tailor line height, and trivival change is needed.
However, such feature might not be needed if vte can deduce a sane base line height for a font, possibly after #347 is resolved.