Initialize XImage struct fully

I was getting sporadic failures from XInitImage (causing the raster
backend to not draw anything).

BUG=skia:

Change-Id: Ied79f2b5354d89c146aca64d70fde8cf9f3ab62e
Reviewed-on: https://skia-review.googlesource.com/9102
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>

tools/viewer/sk_app/unix/RasterWindowContext_unix.cpp

@@ -63,6 +63,7 @@ void RasterWindowContext_xlib::swapBuffers() {
     }
     int bitsPerPixel = pm.info().bytesPerPixel() * 8;
     XImage image;
+    memset(&image, 0, sizeof(image));
     image.width = pm.width();
     image.height = pm.height();
     image.format = ZPixmap;