message: Try harder to not require include path
diff: #include <cassert>
  #include <cstddef>
  -#include <visionaray/math/detail/math.h>
  -#include <visionaray/math/matrix.h>
  -#include <visionaray/math/unorm.h>
  -#include <visionaray/math/vector.h>
  -#include <visionaray/pixel_format.h>
  -#include <visionaray/spectrum.h>
  +#include "../math/detail/math.h"
  +#include "../math/matrix.h"
  +#include "../math/unorm.h"
  +#include "../math/vector.h"
  +#include "../pixel_format.h"
  +#include "../spectrum.h"
  #include "spd/blackbody.h"
  #include "macros.h"
  