[gtk+/wip/baedert/gl] gl renderer: Don't crash if the first op is an opacity one

[Timm Bäder <baedert src gnome org>]

commit 66e35578cf5d1d81e50cb3c452b735e3283e4974
Author: Timm Bäder <mail baedert org>
Date:   Tue Dec 19 16:00:43 2017 +0100

    gl renderer: Don't crash if the first op is an opacity one

 gsk/gl/gskglrenderops.c |   17 +++++++++++++----
 1 files changed, 13 insertions(+), 4 deletions(-)
---
diff --git a/gsk/gl/gskglrenderops.c b/gsk/gl/gskglrenderops.c
index 7b9a6a2..9e2f719 100644
--- a/gsk/gl/gskglrenderops.c
+++ b/gsk/gl/gskglrenderops.c
@@ -236,11 +236,20 @@ ops_set_opacity (RenderOpBuilder *builder,
   if (builder->current_opacity == opacity)
     return opacity;
 
-  last_op = &g_array_index (builder->render_ops, RenderOp, builder->render_ops->len - 1);
-
-  if (last_op->op == OP_CHANGE_OPACITY)
+  if (builder->render_ops->len > 0)
     {
-      last_op->opacity = opacity;
+      last_op = &g_array_index (builder->render_ops, RenderOp, builder->render_ops->len - 1);
+
+      if (last_op->op == OP_CHANGE_OPACITY)
+        {
+          last_op->opacity = opacity;
+        }
+      else
+        {
+          op.op = OP_CHANGE_OPACITY;
+          op.opacity = opacity;
+          g_array_append_val (builder->render_ops, op);
+        }
     }
   else
     {