[smooth] Fix uninitialized memory access. Looks like `ras.span_y' could always be used without initialization. This was never detected by valgrind before because the library-wide `raster_pool' was used for the worker object and `raster_pool' was originally zero'ed. But subsequent reuses of it were using `span_y' uninitialized. With the recent change to not use `render_pool' and allocate worker and buffer on the stack, valgrind now detects this uninitialized access. * src/smooth/ftgrays.c (gray_raster_render): Initialize `ras.span_y'.