LibGfx: Add simple VectorGraphic abstract base class

This is currently very bare-bones with just the size and a few methods
to rasterize/draw the graphic.

Userland/Libraries/LibGfx/CMakeLists.txt

@@ -64,6 +64,7 @@ set(SOURCES
     TextDirection.cpp
     TextLayout.cpp
     Triangle.cpp
+    VectorGraphic.cpp
     WindowTheme.cpp
 )