[Xr] XrSetPattern added

[Carl Worth]

I just committed a long-overdue function to Xr:

	void XrSetPattern(XrState *xrs, XrSurface *pattern);

which allows you to draw using a pattern rather than a solid color.

This should be quite helpful for those who want to do things like
gradients.

I also updated the xrknockout program, (in the xrtest module in CVS),
to use this function to draw the checked background. That also
demonstrates the use of XrSurfaceSetRepeat to allow a repeated
pattern.

One thing still missing from the pattern API is a way to set a
transformation matrix for the pattern. To simplify the API, it may
make sense to expose an XrTransform object which could be used both
for the Xr user->device space transformation as well as surface
transformations.

One complication is that the user->device space matrix only supports
affine transformations while the surface code has support for a full
projective transformation matrix.

Once the surface transformation is in place, it should be possible to
use this to easily and efficiently drawing some gradients.

-Carl

-- 
Carl Worth                                        
USC Information Sciences Institute                      cworth at isi.edu