Add a cairo based framebuffer implementation

In GTK3, GdkImage has been removed completely. The vncimageframebuffer
class cannot thus be used. Replace it with a vnccairoframebuffer
class instead, but keep the old one around in GTK2 builds for ABI
compatability.

* src/vnccairoframebuffer.c, src/vnccairoframebuffer.h,
  src/libgtk-vnc_sym.version: Add new framebuffer impl
  based on Cairo.
* src/Makefile.am: Disable GdkImage based framebuffer on
  GTK3 builds
* src/vncdisplay.c: Always use Cairo based framebuffer