GLX: Fix protocol for glTexSubImage#D

GLX: Fix protocol for glTexSubImage#D

The TexSubImage commands do not have the "NULL image" flag that was
introduced with glTexImage3D.  However, there is a CARD32 pad element
where that flag would be.  Removing the img_null_flag causes the flag
to be removed from the protocol.  This changes the protocol and breaks
everything.

In order to prevent needing to hand-code all of the TexSubImage
functions, a new attribute was added to the param element.  This new
attribute, called "padding," is a boolean flag that selects whether or
not the parameter is a real parameter (default / false) or is protocol
padding (true) that does not appear in the function's parameter list.

This change resulted in a number of changes to other Python scripts.
In almost all cases parameters with the is_padding flag set should not
be emitted.

This patch only changes the the XML, the DTD, and the generator
scripts.  It does NOT include the resulting changes to the generated
code.  Generated code in the X server is also changed by the script /
XML changes in this patch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>