DisplayState interface change

Hi all,
this is the second update of the "DisplayState interface change" series.

The series is now of made of 5 patches:

1) introducing accessors
this is the brainless substitutions patch that changes ds->linesize with ds_get_linesize(ds) and friends everywhere;

2) qemu_reallocz
this patch implements a qemu_reallocz function, that will be used later;

3) remove bgr
the new DisplayState interface does not contain any host specific display detail, it is an abstraction of the backend display, hence we don't need to memorize the bgr flag in DisplayState.
The frontend must be able to handle a bgr display by itself, in fact sdl is perfectly capable of that;

4) DisplayState interface change
this is the big patch that actually changes the interface;

5) vnc improvements
this patch introduces DisplaySurfaces in vnc, simplifying the code.

there are no semantic changes compared to the last update of this series.

Cheers,

Stefano