message: Don't export image subclasses
diff: #include <string>
  -#include "export.h"
  #include "image_base.h"
  namespace visionaray
  @@ -19,15 +18,15 @@ class pnm_image : public image_base
  public:
  // Default constructor.
  - VSNRAY_COMMON_EXPORT pnm_image() = default;
  + pnm_image() = default;
  // Construct image from width, height, format, and data (data is copied).
  - VSNRAY_COMMON_EXPORT pnm_image(int width, int height, pixel_format format, uint8_t const* data);
  + pnm_image(int width, int height, pixel_format format, uint8_t const* data);
  - VSNRAY_COMMON_EXPORT bool load(std::string const& filename);
  + bool load(std::string const& filename);
  // Save pnm image. Options: { "binary", <bool> }
  - VSNRAY_COMMON_EXPORT bool save(std::string const& filename, save_options const& options);
  + bool save(std::string const& filename, save_options const& options);
  };
  