Xnest: Implement xnestModifyPixmapHeader

Xnest variant of ModifyPixmapHeader that creates new Pixmap in parent X
server if it's size is modified from 0x0 to anything bigger.

xnestCreatePixmap doesn't create pixmap in parent X server if it has
dimensions 0x0. If it is later resized and accessed, Xnest will be
aborted with BadDrawable error from parent X server because it will
use XID 0. This happens with ScratchPixmap, for example as used from
XaceCensorImage. Applications using XACE crash Xnest.

Reviewed-by: Adam Jackson <ajax@redhat.com>

hw/xnest/XNPixmap.h

@@ -33,6 +33,8 @@ typedef struct {
 PixmapPtr xnestCreatePixmap(ScreenPtr pScreen, int width, int height,
                             int depth, unsigned usage_hint);
 Bool xnestDestroyPixmap(PixmapPtr pPixmap);
+Bool xnestModifyPixmapHeader(PixmapPtr pPixmap, int width, int height, int depth,
+                             int bitsPerPixel, int devKind, pointer pPixData);
 RegionPtr xnestPixmapToRegion(PixmapPtr pPixmap);
 
 #endif                          /* XNESTPIXMAP_H */