[FontCollector] Add a warning message if a font has variable axes

[moi15moi]

Currently, libass doesn't support variable font: libass/libass#386. This means that when the user works in Aegisub, the font may works because it use the os API, but when the font is muxed into a mkv, it may not be supported. For this reason, in the FontCollector, there should be a warning like: Warning: The font X is a variable font. Libass doesn't properly support those font. This leads to unknown behaviour when the font is muxed to a mkv if a font has variable axes.

Since the FontCollector depend on various API depending of the OS, here are the function that could help us to know if a font is a variable font:

• Windows: IDWriteFontFace5::HasVariations
• macOS: CTFontDescriptorCopyAttribute with kCTFontVariationAttribute as attribute.
• FcPatternGetBool with FC_VARIABLE as object.