message: For linear search, std::vector will suffice
diff: #include <cassert>
  #include <algorithm>
  -#include <map>
  #include <utility>
  +#include <vector>
  #if VSNRAY_HAVE_GLEW
  #include <GL/glew.h>
  @@ -36,7 +36,7 @@ struct format_key
  }
  };
  -static std::map<format_key, pixel_format> gl_formats(
  +static std::vector<std::pair<format_key, pixel_format>> gl_formats(
  {
  #if defined(GL_VERSION_1_1) && GL_VERSION_1_1 // TODO!
  { { GL_R8, GL_UNSIGNED_BYTE, 1 }, PF_R8 },
  