gallium: new draw stage for polygon stipple.

gallium: new draw stage for polygon stipple.

For hardware without native polygon stipple.  Create a 32x32 alpha texture
that encodes the stipple pattern.  Modify the user's fragment program to
sample the texture (with gl_FragCoord) and kill the fragment according to
the texel value.
Temporarily enabled in softpipe driver, replacing the sp_quad_stipple.c step.