vendor: import KillerPDF 1.7.5 source (GPL-3.0) as the base for MMD PDF

Models/ViewTypes.cs

@@ -0,0 +1,20 @@
+namespace KillerPDF
+{
+    // How a document view lays its pages out, and how it fits them to the viewport.
+    //
+    // TOP-LEVEL, not nested in MainWindow. The viewer is a UserControl in KillerPDF.Controls, and
+    // from there a type nested in MainWindow only spells as MainWindow.ViewMode - which would mean
+    // qualifying 91 references for no gain. As top-level types in KillerPDF they resolve
+    // unqualified from KillerPDF.Controls too (a namespace declaration puts its parent namespaces
+    // in scope), so every call site compiles untouched.
+    //
+    // internal, not public: nothing outside the assembly has any business with either.
+
+    /// <summary>Page layout for a document view. RenderPage is Single/TwoPage/Grid only and is
+    /// guarded to no-op in Continuous - see the render pipeline's notes on why the two pipelines
+    /// cannot be mixed.</summary>
+    internal enum ViewMode { Single, Continuous, TwoPage, Grid }
+
+    /// <summary>Automatic fit applied on resize, or None when the user has set a zoom.</summary>
+    internal enum FitMode { None, Width, Page }
+}