Fix softproofing with some internal profiles

The problem is that lcms2 is too smart and not forcing clipping during
softproofing. That might be useful when simulating the effect the
profile alone has, but useless when simulating export to a ldr format.
The workaround is to dump the profile to memory and read it back so that
lcms2 quantizes the TRC.
See https://sourceforge.net/p/lcms/mailman/message/35178784/

src/iop/colorout.c

@@ -552,6 +552,25 @@ void process_sse2(struct dt_iop_module_t *self, dt_dev_pixelpipe_iop_t *piece, c
 }
 #endif
 
+static cmsHPROFILE _make_clipping_profile(cmsHPROFILE profile)
+{
+  cmsUInt32Number size;
+  cmsHPROFILE old_profile = profile;
+  profile = NULL;
+
+  if(old_profile && cmsSaveProfileToMem(old_profile, NULL, &size))
+  {
+    char *data = malloc(size);
+
+    if(cmsSaveProfileToMem(old_profile, data, &size))
+      profile = cmsOpenProfileFromMem(data, size);
+
+    free(data);
+  }
+
+  return profile;
+}
+
 void commit_params(struct dt_iop_module_t *self, dt_iop_params_t *p1, dt_dev_pixelpipe_t *pipe,
                    dt_dev_pixelpipe_iop_t *piece)
 {
@@ -649,16 +668,25 @@ void commit_params(struct dt_iop_module_t *self, dt_iop_params_t *p1, dt_dev_pix
     {
       softproof = dt_colorspaces_get_profile(DT_COLORSPACE_SRGB, "",
                                              DT_PROFILE_DIRECTION_OUT | DT_PROFILE_DIRECTION_DISPLAY)->profile;
-      dt_control_log(_("missing output profile has been replaced by sRGB!"));
-      fprintf(stderr, "missing output profile `%s' has been replaced by sRGB!\n",
-              dt_colorspaces_get_name(out_type, out_filename));
+      dt_control_log(_("missing softproof profile has been replaced by sRGB!"));
+      fprintf(stderr, "missing softproof profile `%s' has been replaced by sRGB!\n",
+              dt_colorspaces_get_name(darktable.color_profiles->softproof_type,
+                                      darktable.color_profiles->softproof_filename));
     }
 
-    /* TODO: the use of bpc should be userconfigurable either from module or preference pane */
-    /* softproof flag and black point compensation */
-    transformFlags |= cmsFLAGS_SOFTPROOFING | cmsFLAGS_NOCACHE | cmsFLAGS_BLACKPOINTCOMPENSATION;
+    // some of our internal profiles are what lcms considers ideal profiles as they have a parametric TRC so
+    // taking a roundtrip through those profiles during softproofing has no effect. as a workaround we have to
+    // make lcms quantisize those gamma tables to get the desired effect.
+    // in case that fails we don't enable softproofing.
+    softproof = _make_clipping_profile(softproof);
+    if(softproof)
+    {
+      /* TODO: the use of bpc should be userconfigurable either from module or preference pane */
+      /* softproof flag and black point compensation */
+      transformFlags |= cmsFLAGS_SOFTPROOFING | cmsFLAGS_NOCACHE | cmsFLAGS_BLACKPOINTCOMPENSATION;
 
-    if(d->mode == DT_PROFILE_GAMUTCHECK) transformFlags |= cmsFLAGS_GAMUTCHECK;
+      if(d->mode == DT_PROFILE_GAMUTCHECK) transformFlags |= cmsFLAGS_GAMUTCHECK;
+    }
   }
 
   /*
@@ -719,6 +747,8 @@ void commit_params(struct dt_iop_module_t *self, dt_iop_params_t *p1, dt_dev_pix
   }
 
   g_free(over_filename);
+  // softproof is never the original but always a copy that went through _make_clipping_profile()
+  dt_colorspaces_cleanup_profile(softproof);
 }
 
 void init_pipe(struct dt_iop_module_t *self, dt_dev_pixelpipe_t *pipe, dt_dev_pixelpipe_iop_t *piece)