gui_fader: restore previous dither-matrix indexing

The dither matrix is not perfectly random but provides a kind of
pseudo-random pattern that fits the original dithering approach with
global coordinate based indexing. If the matrix is offset by one line
each 16 pixels the pattern becomes visible as alternating light and dark
columns.

Follow-up commit to "gui_fader: precompute dithered 16x16 tile".

Issue #5501

repos/gems/src/server/gui_fader/alpha_dither_painter.h

@@ -69,7 +69,7 @@ struct Alpha_dither_painter
 			unsigned const y_offset = y << 4;
 
 			for (dst = dst_line, w = clipped.w(); w--; dst++, x++)
-				dst->pixel = tile.v[(y_offset + x) & 0xff];
+				dst->pixel = tile.v[(y_offset + (x & 0xf)) & 0xff];
 		}
 	}