gallium/indices: don't expand prim-type for 8-bit indices

Expanding the primitive-type has two undesirable effects:

1. It breaks primitive-restart. This is possible to fix by explicitly
   handling primitive-restart in more conversion routines. But
   u_indices_gen.py is kind of a mess, so it's not trivial as-is.
2. It changes the reported gl_VertexID.

While it might be possible to work around this in each driver, it seems
better to avoid this when we can.

Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Reviewed-by: Gert Wollny <gert.wollny@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5976>

src/gallium/auxiliary/indices/u_indices.c

@@ -45,6 +45,19 @@ static void translate_memcpy_uint( const void *in,
    memcpy(out, &((int *)in)[start], out_nr*sizeof(int));
 }
 
+static void translate_byte_to_ushort( const void *in,
+                                      unsigned start,
+                                      UNUSED unsigned in_nr,
+                                      unsigned out_nr,
+                                      UNUSED unsigned restart_index,
+                                      void *out )
+{
+   uint8_t *src = (uint8_t *)in + start;
+   uint16_t *dst = out;
+   while (out_nr--) {
+      *dst++ = *src++;
+   }
+}
 
 /**
  * Translate indexes when a driver can't support certain types
@@ -98,14 +111,14 @@ u_index_translator(unsigned hw_mask,
    out_idx = out_size_idx(*out_index_size);
 
    if ((hw_mask & (1<<prim)) &&
-       in_index_size == *out_index_size &&
        in_pv == out_pv)
    {
-      /* Index translation not really needed */
       if (in_index_size == 4)
          *out_translate = translate_memcpy_uint;
-      else
+      else if (in_index_size == 2)
          *out_translate = translate_memcpy_ushort;
+      else
+         *out_translate = translate_byte_to_ushort;
 
       *out_prim = prim;
       *out_nr = nr;