RENDER extension implementation for python-xlib
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This is a work-in-progress; the interface can and will change.

To test this, apply integrate-render.patch against r157 of the
python-xlib trunk and copy render.py to the ext/ directory under Xlib.
There's also object-fix.patch which fixes Object.check_value() and
simplifies the other methods a bit, which makes the Fixed type in the
render extension serialize correctly when used with Object.

The support for multiple fields in LengthOf that was added in r157 is
needed for render.py to work, although it is not strictly necessary,
since the feature is only used in requests. Maybe the feature could
add a check that all the referenced fields are the same size?
(Kind of like FixedList checks that the value is the right size)