message: fix setting spec constants in gl backend
  a failure could happen if a comment contained the word `#extension`
diff: @@ -239,9 +239,11 @@ std::array<std::string_view, 2> OpenGLProgram::splitShaderSource(std::string_vie
  auto start = source.find("#version");
  assert_invariant(start != std::string_view::npos);
  - auto pos = source.rfind("#extension");
  + auto pos = source.rfind("\n#extension");
  if (pos == std::string_view::npos) {
  pos = start;
  + } else {
  + ++pos;
  }
  auto eol = source.find('\n', pos) + 1;
  