big-requests support only partially implemented

Hi,

 affects clx
 importance medium
 tag big-requests extension

Although the big-request extension is properly initialized and enabled,
it never gets used; all length computations use the X11 protocol 16-bit
length field, with (if I recall correctly) no error checking.

In my mail archive I have code from Richard Kreuter implementing a
proof-of-concept use of big-requests, against a slightly different
version of the client extension support.

Christophe