cegui doesn't include stddef required for ptrdiff_t type, we
include it for it.

This stops the client from dropping CEGUI.log files into the cwd all
the time, and stops it from crashing when the cwd is not writable
(rhbz#650253).

Now that Application::hide_me actually does what the name suggests
(hide the entire client, ie all client windows), the gui using it to
not show the gui layer leads to the entire client disappearing when
one presses close in the GUI or dismisses a GUI dialog. This patch makes
the GUI code call hide_gui instead of hide_me, fixing this.

letancy -> latency
compund -> compound
SpicedSubMessage -> SpiceSubMessage
modifaiers -> modifiers
massage -> message
outgoiong -> outgoing
AlphaBlnd -> AlphaBlend
remoth -> remote
modifires -> modifiers
secore -> secure

This makes the CEGUI dependency optional and off by default. Restoring
previous behaviour of exiting on disconnect if disabled.

Also prints a simpler error to stderr for WARN or above so that
we print something on the commandline if something go wrong.

The command line option is renamed from "cairo" to "sw", and
similarly all filenames and types from Cairo to Sw (and similar).

In order to be able to support 16bit canvases on 32bit screens and 32bit
canvases on 16bit screens we need to handle format conversion when drawing
RedPixmaps.

The way this works now for X11 is that we only have one PIXELS_SOURCE_TYPE
for pixmaps, which always has a pixman_image_t for the data, but additionally
it has an XImage (shared mem or not) if the screen the pixmap was created
for (i.e. an explicit one or the default screen) has the same format as
the pixmap.

When we draw a pixmap on a drawable we have two variants. If the pixmap
has a XImage and it matches the format of the target drawable then we
just X(Shm)PutImage it to the drawable. If the formats differ, then we
create a temporary XImage and convert into that before drawing it to
the screen.

Right now this is a bit inefficient, because we always allocate a new
temporary image when converting. We want to add some caching here, but
at least this lets things work again.