Codechange: Use ProviderManager interface to register FontCache factories.

This removes use of #ifdefs to select the appropriate loader, and also replaces FontCache self-registration.

src/fontcache/spritefontcache.cpp

@@ -151,3 +151,20 @@ bool SpriteFontCache::GetDrawGlyphShadow()
 {
 	return false;
 }
+
+class SpriteFontCacheFactory : public FontCacheFactory {
+public:
+	SpriteFontCacheFactory() : FontCacheFactory("sprite", "Sprite font provider") {}
+
+	std::unique_ptr<FontCache> LoadFont(FontSize fs, FontType fonttype) override
+	{
+		if (fonttype != FontType::Sprite) return nullptr;
+
+		return std::make_unique<SpriteFontCache>(fs);
+	}
+
+private:
+	static SpriteFontCacheFactory instance;
+};
+
+/* static */ SpriteFontCacheFactory SpriteFontCacheFactory::instance;