softpipe: return alpha=1 in shadow_compare() function

The alpha value wasn't set at all before so we got unpredictable results.

Note that we don't currently obey GL_DEPTH_TEXTURE_MODE in the state
tracker.  For now, we return the result in the default mode (r,r,r,1).

src/gallium/drivers/softpipe/sp_tex_sample.c

@@ -744,7 +744,9 @@ shadow_compare(uint compare_func,
       break;
    }
 
+   /* XXX returning result for default GL_DEPTH_TEXTURE_MODE = GL_LUMINANCE */
    rgba[0][j] = rgba[1][j] = rgba[2][j] = (float) k;
+   rgba[3][j] = 1.0F;
 }