Create a proper interface base class for the viewport's framebuffer

This allows us to gracefully fall back to the FLTK code in case the
platform specific code cannot be used.
diff --git a/vncviewer/CMakeLists.txt b/vncviewer/CMakeLists.txt
index c1facc6..9b815a3 100644
--- a/vncviewer/CMakeLists.txt
+++ b/vncviewer/CMakeLists.txt
@@ -7,9 +7,11 @@
   menukey.cxx
   CConn.cxx
   DesktopWindow.cxx
+  FLTKPixelBuffer.cxx
   UserDialog.cxx
   ServerDialog.cxx
   OptionsDialog.cxx
+  PlatformPixelBuffer.cxx
   Viewport.cxx
   parameters.cxx
   keysym2ucs.c