Add extended shared memory attach structure

This provides an extended version of `X_ShmAttachFd`, called
`X_ShmAttachFdExt`, which allows the client to specify two additional
parameters:

1. The size of the mapping, in bytes.
2. The offset that should be passed to `mmap`, in bytes.

The intended use-case for this extension is when one needs to map
a drawable from a character special device, but it can also be
used with regular files if one needs to map with a nonzero offset.
Qubes OS needs the X server to map the `/dev/xen/gntdev` character
device, which represents memory shared from a different virtual
machine.  This is currently accomplished with an `LD_PRELOAD`’d
shared library that overrides `shmat`, `shmctl`, and `shmdt`, and
I have written a version that instead overrides `fstat` and `mmap`.
With this extension, the preloaded shared library will not be required.

Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>

include/X11/extensions/shmproto.h

@@ -45,6 +45,7 @@ in this Software without prior written authorization from The Open Group.
 #define X_ShmCreatePixmap		5
 #define X_ShmAttachFd                   6
 #define X_ShmCreateSegment              7
+#define X_ShmAttachFdExt                8
 
 typedef struct _ShmQueryVersion {
     CARD8	reqType;		/* always ShmReqCode */
@@ -220,6 +221,21 @@ typedef struct {
 /* File descriptor is passed with this reply */
 #define sz_xShmCreateSegmentReply	32
 
+/* Version 1.3 additions */
+typedef struct _ShmAttachFdExt {
+    CARD8	reqType;	/* always ShmReqCode */
+    CARD8	shmReqType;	/* always X_ShmAttachFdExt */
+    CARD16	length;
+    ShmSeg	shmseg;
+    BOOL	readOnly;
+    BYTE	pad0;
+    CARD16	pad1;
+    CARD32	size;		/* size of the buffer */
+    CARD64	offset;		/* offset to pass to mmap() */
+} xShmAttachFdExtReq;
+/* File descriptor is passed with this request */
+#define sz_xShmAttachFdExtReq 32
+
 #undef ShmSeg
 #undef Drawable
 #undef VisualID