Fixed #43 CTFontCreateWithName() warning on iOS 13

[ElfSundae]

Fixed #43

CTFontCreateWithName(".SFUI-Regular") 在 iOS 13 之前会返回 .SFUIText-Regular 这个字体，iOS 13 会返回 Times New Roman 这个字体，并且会输出警告：

CoreText performance note: Client called CTFontCreateWithName() using name ".SFUI-Regular" and got font with PostScript name "TimesNewRomanPSMT". For best performance, only use PostScript names when calling this API.

Ref from: Cocoanetics/DTCoreText#1168 (comment)

This issue behaves as intended.
As mentioned in numerous WWDC sessions, dot-prefixed font names are not to be directly used.
I would also note that UIFont and CTFont are toll-free bridged, so they can be used interchangeably with casts.
Please update your feedback report to let us know if this is still an issue for you.

苹果意思是不要直接使用 (.) 开头的字体名。

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修复方案 1： 使用 CTFontCreateWithFontDescriptor 创建 CTFont
修复方案 2：因为 UIFont 和 CTFont 是 toll-free bridged 的，UIFont 其实就是 CTFont，所以可以在任何需要使用 CTFont 的地方类型转换 UIFont 即可： (__bridge CTFontRef)self.font