cppcanvas: avoid -Werror=deprecated-copy (GCC trunk towards GCC 9)

...by explicitly defaulting the copy/move functions (and, where needed in turn,
also a default ctor) for classes that have a user-declared dtor that does
nothing other than an implicitly-defined one would do, but needs to be user-
declared because it is virtual and potentially serves as a key function to
emit the vtable, or is non-public, etc.

Change-Id: Ifa6e6f71f73b9d0a577de1a90cbf3a8c969ac062
Reviewed-on: https://gerrit.libreoffice.org/58073
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

include/cppcanvas/canvas.hxx

@@ -69,6 +69,12 @@ namespace cppcanvas
             ANTIALIASING_EXTRA_SIZE=2
         };
 
+        Canvas() = default;
+        Canvas(Canvas const &) = default;
+        Canvas(Canvas &&) = default;
+        Canvas & operator =(Canvas const &) = default;
+        Canvas & operator =(Canvas &&) = default;
+
         virtual ~Canvas() {}
 
         virtual void                             setTransformation( const ::basegfx::B2DHomMatrix& rMatrix ) = 0;