字体相关概念

字体基本概念
https://blog.youkuaiyun.com/flyeek/article/details/43970193
深入理解字体：参考线、line-box（行框）、content-area
https://www.cnblogs.com/guyuedashu/p/12499328.html
Android font, 字体全攻略
https://www.jianshu.com/p/35328f7ac54a
Meaning of top, ascent, baseline, descent, bottom, and leading in Android’s FontMetrics
https://stackoverflow.com/questions/27631736/meaning-of-top-ascent-baseline-descent-bottom-and-leading-in-androids-font/27631737#27631737
Paint.FontMetrics
https://developer.android.google.cn/reference/android/graphics/Paint.FontMetrics

Ascent - Descent: 字体实际大小, 背景填充区域
font-size: 字体相对大小
line-height: top - bottom