System font API (#5393)

* System font API

This is a new API that allows operating system fonts to be loaded by the engine and used by content.

Fonts are provided in a flat list exposing all the relevant metadata. They are loaded from disk with a Load call.

Initial implementation is only for Windows DirectWrite.

* Load system fonts as memory mapped files if possible.

This allows sharing the font file memory with other processes which is always good.

* Use ArrayPool to reduce char array allocations

* Disable verbose logging

* Implement system font support on Linux via Fontconfig

* Implement macOS support

* Add "FREEDESKTOP" define constant

This is basically LINUX || FREEBSD. Though FreeBSD currently gets detected as LINUX too. Oh well.

* Compile out Fontconfig and CoreText system font backends when not on those platforms

* Don't add Fontconfig package dep on Mac/Windows

* Allow disabling system font support via CVar

Cuz why not.

MSBuild/Robust.DefineConstants.targets

@@ -13,7 +13,7 @@
     </When>
     <Otherwise>
       <PropertyGroup>
-        <DefineConstants>$(DefineConstants);LINUX;UNIX</DefineConstants>
+        <DefineConstants>$(DefineConstants);LINUX;UNIX;FREEDESKTOP</DefineConstants>
       </PropertyGroup>
     </Otherwise>
   </Choose>

MSBuild/Robust.Platform.props

@@ -31,5 +31,6 @@
     <Python>python3</Python>
     <Python Condition="'$(ActualOS)' == 'Windows'">py -3</Python>
     <UseSystemSqlite Condition="'$(TargetOS)' == 'FreeBSD'">True</UseSystemSqlite>
+    <IsFreedesktop Condition="'$(TargetOS)' == 'FreeBSD' Or '$(TargetOS)' == 'Linux'">True</IsFreedesktop>
   </PropertyGroup>
 </Project>