gsksltype: Add gsk_sl_type_get_index_stride()

This allows operations on GskSlValues that are arrays, because one can
just do code like:
  data = gsk_sl_value_get_data (value);
  stride = gsk_sl_value_get_index_stride (type);
  for (i = 0; i < gsk_sl_value_get_length (type); i++)
    {
      do_stuff_with_data (data + i * stride);
    }

gsk/gsksltype.c

@@ -44,6 +44,7 @@ struct _GskSlTypeClass {
   const char *          (* get_name)                            (const GskSlType     *type);
   GskSlScalarType       (* get_scalar_type)                     (const GskSlType     *type);
   GskSlType *           (* get_index_type)                      (const GskSlType     *type);
+  gsize                 (* get_index_stride)                    (const GskSlType     *type);
   guint                 (* get_length)                          (const GskSlType     *type);
   gsize                 (* get_size)                            (const GskSlType     *type);
   gboolean              (* can_convert)                         (const GskSlType     *target,
@@ -349,6 +350,12 @@ gsk_sl_type_scalar_get_index_type (const GskSlType *type)
   return NULL;
 }
 
+static gsize
+gsk_sl_type_scalar_get_index_stride (const GskSlType *type)
+{
+  return 0;
+}
+
 static guint
 gsk_sl_type_scalar_get_length (const GskSlType *type)
 {
@@ -472,6 +479,7 @@ static const GskSlTypeClass GSK_SL_TYPE_SCALAR = {
   gsk_sl_type_scalar_get_name,
   gsk_sl_type_scalar_get_scalar_type,
   gsk_sl_type_scalar_get_index_type,
+  gsk_sl_type_scalar_get_index_stride,
   gsk_sl_type_scalar_get_length,
   gsk_sl_type_scalar_get_size,
   gsk_sl_type_scalar_can_convert,
@@ -522,6 +530,14 @@ gsk_sl_type_vector_get_index_type (const GskSlType *type)
   return gsk_sl_type_get_scalar (vector->scalar);
 }
 
+static gsize
+gsk_sl_type_vector_get_index_stride (const GskSlType *type)
+{
+  const GskSlTypeVector *vector = (const GskSlTypeVector *) type;
+
+  return scalar_infos[vector->scalar].size;
+}
+
 static guint
 gsk_sl_type_vector_get_length (const GskSlType *type)
 {
@@ -636,6 +652,7 @@ static const GskSlTypeClass GSK_SL_TYPE_VECTOR = {
   gsk_sl_type_vector_get_name,
   gsk_sl_type_vector_get_scalar_type,
   gsk_sl_type_vector_get_index_type,
+  gsk_sl_type_vector_get_index_stride,
   gsk_sl_type_vector_get_length,
   gsk_sl_type_vector_get_size,
   gsk_sl_type_vector_can_convert,
@@ -687,6 +704,14 @@ gsk_sl_type_matrix_get_index_type (const GskSlType *type)
   return gsk_sl_type_get_vector (matrix->scalar, matrix->rows);
 }
 
+static gsize
+gsk_sl_type_matrix_get_index_stride (const GskSlType *type)
+{
+  const GskSlTypeMatrix *matrix = (const GskSlTypeMatrix *) type;
+
+  return scalar_infos[matrix->scalar].size * matrix->rows;
+}
+
 static guint
 gsk_sl_type_matrix_get_length (const GskSlType *type)
 {
@@ -802,6 +827,7 @@ static const GskSlTypeClass GSK_SL_TYPE_MATRIX = {
   gsk_sl_type_matrix_get_name,
   gsk_sl_type_matrix_get_scalar_type,
   gsk_sl_type_matrix_get_index_type,
+  gsk_sl_type_matrix_get_index_stride,
   gsk_sl_type_matrix_get_length,
   gsk_sl_type_matrix_get_size,
   gsk_sl_type_matrix_can_convert,
@@ -1124,6 +1150,12 @@ gsk_sl_type_get_index_type (const GskSlType *type)
   return type->class->get_index_type (type);
 }
 
+gsize
+gsk_sl_type_get_index_stride (const GskSlType *type)
+{
+  return type->class->get_index_stride (type);
+}
+
 guint
 gsk_sl_type_get_length (const GskSlType *type)
 {

gsk/gsksltypeprivate.h

@@ -52,6 +52,7 @@ gboolean                gsk_sl_type_is_matrix                   (const GskSlType
 const char *            gsk_sl_type_get_name                    (const GskSlType     *type);
 GskSlScalarType         gsk_sl_type_get_scalar_type             (const GskSlType     *type);
 GskSlType *             gsk_sl_type_get_index_type              (const GskSlType     *type);
+gsize                   gsk_sl_type_get_index_stride            (const GskSlType     *type);
 guint                   gsk_sl_type_get_length                  (const GskSlType     *type);
 gsize                   gsk_sl_type_get_size                    (const GskSlType     *type);
 gboolean                gsk_sl_scalar_type_can_convert          (GskSlScalarType      target,

gsk/gskslvalue.c

@@ -113,8 +113,8 @@ gsk_sl_value_new_convert (GskSlValue *source,
       result = gsk_sl_value_new (new_type);
       sdata = source->data;
       ddata = result->data;
-      sstride = gsk_sl_type_get_size (source->type) / n;
-      dstride = gsk_sl_type_get_size (new_type) / n;
+      sstride = gsk_sl_type_get_index_stride (source->type);
+      dstride = gsk_sl_type_get_index_stride (new_type);
       for (i = 0; i < n; i++)
         {
           gsk_sl_scalar_type_convert_value (gsk_sl_type_get_scalar_type (new_type),