Fix memleak of xcb_image_t
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