[gegl] long-shadow: fix buffer overflow; add "needs-alpha" key

[Ell <ell src gnome org>]

commit d13895edc11fcd1c88a404a6401e2b9dc41d45e4
Author: Ell <ell_se yahoo com>
Date:   Mon Jul 30 13:58:10 2018 -0400

    long-shadow: fix buffer overflow; add "needs-alpha" key

 operations/workshop/long-shadow.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)
---
diff --git a/operations/workshop/long-shadow.c b/operations/workshop/long-shadow.c
index 0ac702072..6438815f0 100644
--- a/operations/workshop/long-shadow.c
+++ b/operations/workshop/long-shadow.c
@@ -701,12 +701,9 @@ init_buffers (Context    *ctx,
   else
     {
       ctx->input_row0  = ctx->input_row  + 4 * (ctx->area.width - 1);
-      ctx->output_row0 = ctx->output_row + 4 * (ctx->area.width - 1);
+      ctx->output_row0 = ctx->output_row + 4 * (ctx->roi.width  - 1);
 
       ctx->row_step    = -4;
-
-      if (ctx->output_row == ctx->input_row)
-        ctx->output_row0 -= 4 * (ctx->roi.x - ctx->area.x);
     }
 
   if (ctx->options.style == GEGL_LONG_SHADOW_STYLE_FINITE)
@@ -1099,6 +1096,7 @@ gegl_op_class_init (GeglOpClass *klass)
     "name",        "gegl:long-shadow",
     "title",       _("Long Shadow"),
     "categories",  "light",
+    "needs-alpha", "true",
     "description", _("Creates a long-shadow effect"),
     NULL);
 }