As a preliminary step for working on issue #11, this patch revisits the
'Framebuffer::info' RPC call. Instead of using C-style out paramters,
the new 'mode()' RPC call returns the mode information as an object of
type 'Mode'. Consequently, mode-specific functions such as
'bytes_per_pixel' have been moved to the new 'Framebuffer::Mode' class.
The probing and I/O resource allocation is done only once at the
creation time of the first session. When closing and re-opening the
session, the '_device' object is simply reused. This patch fixes#92.
This patch prevents 'fb_sdl' from wrongly using SDL headers located in
the libports repository. We have to make sure to use the headers
installed on the host system.
