[sfnt] Handle fonts without layer list in 'COLR' v1.

'COLR' v1 fonts do not necessarily need to have a layer list; for
this reason, 'fontTools' recently started generating fonts in a way
that drops the layer list if there are no layers in it.  This
results in the layer list offset becoming zero, which FreeType
treated as an invalid table.  Fix that and handle the case for layer
list offset being 0.  This slightly changes how we need to calculate
the starting offset for paints.

* src/sfnt/ttcolr.c (tt_face_load_colr): Handle case of layer list
offset being zero without outright rejecting table.

2 files changed, 39 insertions, 13 deletions

diff --git a/ChangeLog b/ChangeLog
index a1de34602..eccf4a1c7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,18 @@
+2021-06-30  Dominik Röttsches  <drott@chromium.org>
+
+	[sfnt] Handle fonts without layer list in 'COLR' v1.
+
+	'COLR' v1 fonts do not necessarily need to have a layer list; for
+	this reason, 'fontTools' recently started generating fonts in a way
+	that drops the layer list if there are no layers in it.  This
+	results in the layer list offset becoming zero, which FreeType
+	treated as an invalid table.  Fix that and handle the case for layer
+	list offset being 0.  This slightly changes how we need to calculate
+	the starting offset for paints.
+
+	* src/sfnt/ttcolr.c (tt_face_load_colr): Handle case of layer list
+	offset being zero without outright rejecting table.
+
 2021-06-30  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
 	* src/raster/ftraster.c (Render_Single_Pass): Simplify `band_stack'.
diff --git a/src/sfnt/ttcolr.c b/src/sfnt/ttcolr.c
index 1e297ac8c..34accddb3 100644
--- a/src/sfnt/ttcolr.c
+++ b/src/sfnt/ttcolr.c
@@ -186,24 +186,35 @@
 
       layer_offset_v1 = FT_NEXT_ULONG( p );
 
-      if ( !layer_offset_v1 || layer_offset_v1 >= table_size )
+      if ( layer_offset_v1 >= table_size )
         goto InvalidTable;
 
-      p1            = (FT_Byte*)( table + layer_offset_v1 );
-      num_layers_v1 = FT_PEEK_ULONG( p1 );
+      if ( layer_offset_v1 )
+      {
+        p1            = (FT_Byte*)( table + layer_offset_v1 );
+        num_layers_v1 = FT_PEEK_ULONG( p1 );
 
-      if ( num_layers_v1 * LAYER_V1_LIST_PAINT_OFFSET_SIZE >
-             table_size - layer_offset_v1 )
-        goto InvalidTable;
+        if ( num_layers_v1 * LAYER_V1_LIST_PAINT_OFFSET_SIZE >
+               table_size - layer_offset_v1 )
+          goto InvalidTable;
 
-      colr->num_layers_v1 = num_layers_v1;
-      colr->layers_v1     = p1;
+        colr->num_layers_v1 = num_layers_v1;
+        colr->layers_v1     = p1;
 
-      colr->paints_start_v1 =
-        FT_MIN( colr->base_glyphs_v1 +
-                  colr->num_base_glyphs_v1 * BASE_GLYPH_PAINT_RECORD_SIZE,
-                colr->layers_v1 +
-                  colr->num_layers_v1 * LAYER_V1_LIST_PAINT_OFFSET_SIZE );
+        colr->paints_start_v1 =
+            FT_MIN( colr->base_glyphs_v1 +
+                    colr->num_base_glyphs_v1 * BASE_GLYPH_PAINT_RECORD_SIZE,
+                    colr->layers_v1 +
+                    colr->num_layers_v1 * LAYER_V1_LIST_PAINT_OFFSET_SIZE );
+      }
+      else
+      {
+        colr->num_layers_v1   = 0;
+        colr->layers_v1       = 0;
+        colr->paints_start_v1 =
+          colr->base_glyphs_v1 +
+          colr->num_base_glyphs_v1 * BASE_GLYPH_PAINT_RECORD_SIZE;
+      }
     }
 
     colr->base_glyphs = (FT_Byte*)( table + base_glyph_offset );