backends: X11: Fix window bitmap buffer init

The w and h may be modified upon the window creation so we must use
wreq.w and wreq.h instead of the user requested w and h.

This fixes bug for X11 backend when root window is used and the image
size was wrongly initialized to whatever user supplied instead of the
root window size.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
spiv: Fix image cache limits.

The problem was that one part of the code was expecting the limits as
kilo-bytes while the second as bytes. (I'm very ashamed that I have done
such lousy mistake.)

Resulting in excessively large image cache limit (larger than actual
ram) which triggered swapping after large number of images was viewed.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>