修复平滑滚动下文本渲染错位的问题

[Richasy]

Close #8

epub.js 中对滚动的计算是增量的。这导致在滚动动画未结束时再次滚动，会在前次滚动的位置上再加一个偏移值，从而让实际结果和预期不符。

PR type

What is the purpose of this PR?

• Bug fix

What is the current behavior?

在滚动动画未结束时再次翻页，渲染会错位

What is the new behavior?

滚动时不使用增量计算，而是定值偏移，以避免滚动错位

PR checklist

Please check that your PR meets the following requirements:

• [x] App successfully launched
• [x] File headers have been added to all source files
• [x] NOT Contains breaking updates

Remark

终于发展到被迫改epub.js的源码了 o(TヘTo)