The font module does not work well with fontconfig on Linux.

[林kurn]

Here are the problems I found, and here is a patch that may fix them.

1. The font manager uses the default logical font as the fallback font.
On Linux systems, we can use fontconfig to get a matching list of fonts.
So, I added a new fallback method, FONTCONFIGURATION_FALLBACK, and
implemented it for Linux systems using fontconfig.

2. The FileFont object have no fallback ability when rendering.
I make the FileFont class implement the FontSubstitution interface and
reuse the fallback method on first.
This enhances text display on Linux systems that frequently use FileFont
objects, reduce the inability to display characters in multiple languages
when using physical fonts.

I know my patch still requires improvement, and I haven't adapted it for
Windows and Mac. But I hope this will lead to further discussion,
eventually we can improve these problems.
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