Use xcb_image_destroy instead of free, fixes this ASAN report:

    Direct leak of 8294432 byte(s) in 1 object(s) allocated from:
        ...
        #5 0x7f11f9d71938 in xcb_image_get (/usr/lib/libxcb-image.so.0+0x1938)
        #6 0x452612 in X11ImageGrabber::getWindowPixmap(unsigned int, bool) src/PlatformBackends/X11ImageGrabber.cpp:296
        #7 0x45437c in X11ImageGrabber::grabFullScreen() src/PlatformBackends/X11ImageGrabber.cpp:418
        #8 0x432bdd in ImageGrabber::doImageGrab() src/PlatformBackends/ImageGrabber.cpp:90
        ...

REVIEW: 129137