message: Fix glTFViewer after shaderc update
  Fragment shader outputs now require a location qualifier, that
  glTFViewer was missing in its shaderc.
diff: @@ -208,7 +208,7 @@ namespace {
  layout(location = 0) in vec3 v_normal;
  layout(location = 1) in vec2 v_texcoord;
  - out vec4 fragcolor;
  + layout(location = 0) out vec4 fragcolor;
  void main() {
  const vec3 lightdir = normalize(vec3(-1, -2, 3));
  @@ -224,7 +224,7 @@ namespace {
  layout(location = 0) in vec3 v_normal;
  layout(location = 1) in vec2 v_texcoord;
  - out vec4 fragcolor;
  + layout(location = 0) out vec4 fragcolor;
  void main() {
  const vec3 lightdir = normalize(vec3(-1, -2, 3));
  