After experimenting with uharfbuzz for a while, I found out it was hard
to handle top-to-bottom texts, so I gave up an idea to put an ascender
or a descender value in the arguments. Instead, I simply expose 'offset',
'width' and 'height', which is way more straightforward than the
previous design.
In addition, 'contain' option is added to easily compensate and render
glyphs such as combining accents or excessively tall letters.
