[Xcb] XML description of GLX conditional reply lists

[Jamey Sharp]

I was just looking again at our GLX protocol description. Several GLX
requests currently aren't expressed well. A simple change to the C code
lets XCB be more natural, while still backward compatible with XCB 1.0.
But what description should go in the XML?

Here's the challenge. If GetBooleanv, for example, has only one BOOL to
return, then it returns it in a fixed spot in the reply. Otherwise, it
treats that fixed spot as padding and uses a list instead. Currently, a
caller needs code like this to read the reply: [1]

	if(xcb_glx_get_booleanv_data_length(reply) != 1)
		result = xcb_glx_get_booleanv_data(reply);
	else
		result = &reply->datum;

This pattern should move into xcb_glx_get_booleanv_data and friends.
Such a change doesn't break ABI or API, because the behavior of
xcb_glx_get_booleanv_data only changes when length is 1, and in that
case the current implementation returns a pointer to unallocated memory.

So I know where I want to get to in the generated code, but I'm not sure
how to get there in the XML descriptions or the XSLT. Ideas?

--Jamey

[1] Jeremy, I think the Mesa XCB port is buggy in these requests. If
data_length is 0, then Mesa/XCB looks at datum; but if there was one
item, my reading of server source says data_length would be 1. I think
you were confused because on Xlib Mesa checks that reply.length is
non-zero, but that's not the same as the field that XCB calls "n".
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