Adds set_dash support.
Changes to Quad.

Removes the (misleading) x, y, width, and height members from
sobek.Quad. Further, performs proper bounds expansion.
Fixes a pretty serious Uniform bug.

Instead of checking for boolean false when accepting Uniform values,
check that the value doesn't equal None exactly; 0, 0.0, and False are
all boolean false, but valid Uniform values!
Changed to collections.abc.MutableSequence.

Instead of inherting from Sequence, we not support a "mutable" sequence
API for sobek.Attributes and sobek.Node.
Changes init_cairo to create_surface.
Adds explicit path fixes to sys.path.

The examples can now properly find an "inplace" sobek module in the
parent directory.