Move PostPromotion modification functions to PostBuild

These changes are done to ensure that `IFontHandle.Lock` will be guaranteed to obtain a fully built font that will not be modified any further (unless `PostPromotion` is being used for modifying fonts, which should not be done by clients.)

* Moved `CopyGlyphsAcrossFonts` and `BuildLookupTable` from `PostPromotion` to `PostBuild` build toolkit.
* `IFontAtlasBuildToolkit`: Added `GetFont` to enable retrieving font corresponding to a handle being built.
* `InterfaceManager`: Use `OnPostBuild` for copying glyphs from Mono to Default.
* `FontAtlasBuildStep`:
    * Removed `Invalid` to prevent an unnecessary switch-case warnings.
    * Added contracts on when `IFontAtlas.BuildStepChanged` will be called.

Dalamud/Interface/ManagedFontAtlas/IFontAtlasBuildToolkitPostBuild.cs

@@ -23,4 +23,28 @@ public interface IFontAtlasBuildToolkitPostBuild : IFontAtlasBuildToolkit
     /// <param name="disposeOnError">Dispose the wrap on error.</param>
     /// <returns>The texture index.</returns>
     int StoreTexture(IDalamudTextureWrap textureWrap, bool disposeOnError);
+
+    /// <summary>
+    /// Copies glyphs across fonts, in a safer way.<br />
+    /// If the font does not belong to the current atlas, this function is a no-op.
+    /// </summary>
+    /// <param name="source">Source font.</param>
+    /// <param name="target">Target font.</param>
+    /// <param name="missingOnly">Whether to copy missing glyphs only.</param>
+    /// <param name="rebuildLookupTable">Whether to call target.BuildLookupTable().</param>
+    /// <param name="rangeLow">Low codepoint range to copy.</param>
+    /// <param name="rangeHigh">High codepoing range to copy.</param>
+    void CopyGlyphsAcrossFonts(
+        ImFontPtr source,
+        ImFontPtr target,
+        bool missingOnly,
+        bool rebuildLookupTable = true,
+        char rangeLow = ' ',
+        char rangeHigh = '\uFFFE');
+
+    /// <summary>
+    /// Calls <see cref="ImFontPtr.BuildLookupTable"/>, with some fixups.
+    /// </summary>
+    /// <param name="font">The font.</param>
+    void BuildLookupTable(ImFontPtr font);
 }