Initialize MinikinPaint members

We are stack-allocating MinikinPaint objects in Minikin clients, and
without a constructor adding new members to the struct cannot be done
without updating all clients (only one right now!).

Change-Id: I4170f16498bb6b07cb795495011aca58087ed0bd

engine/src/flutter/include/minikin/MinikinFont.h

@@ -30,6 +30,8 @@ class MinikinFont;
 // Possibly move into own .h file?
 // Note: if you add a field here, also update LayoutCacheKey
 struct MinikinPaint {
+    MinikinPaint() : font(0), size(0), scaleX(0), skewX(0), letterSpacing(0), paintFlags(0),
+            fakery() { }
     MinikinFont *font;
     float size;
     float scaleX;