quic_usr_more_lines_unstable() assumes it can allways copy a complete
scanline. Well, it can't. In case the screen rectangle which needs
updating has an x-offset greater than zero *and* includes the last
scanline of the screen it will overflow the source buffer by
x-offset * bytes-per-pixel bytes.
Instead of having two virtualizations of the canvas we push the
virtualization into the canvas code itself. This not only avoids
the duplication of this code, it also makes the exposed API for the
canvas much smaller (in terms of exported API).
It also lets us use the virtualization to implement basic support
for operations in canvas_base which is then overridden by each canvas
implementation.
pixman_region32_t is an efficient well tested region implementation (its
the one used in X) that we already depend on via pixman and use in
some places. No need to have a custom region implementation.
Instead of passing a bunch of function pointer and an opaque
pointer we make a real type and add a vtable pointer to it.
This means we can simplify all the canvas constructors, etc.
Increase client timeout in order to prevent unnecessary
disconnecting of client while the connection is over WAN.
Tested by changing WinXP resolution (with desktop background) while
connecting over WAN (1.5Mbit 150Kbit)
