Correct handling IOP_CS_RAW colorspace conversions

For non-raw images we can simply avoid the IOP_CS_RGB to IOP_CS_RAW colorspace conversions
completely. This avoids
- stupid fallbacks to CPU code if running OpenCL
- costly copy of data

src/common/iop_profile.c

@@ -634,6 +634,10 @@ static inline void _transform_matrix(struct dt_iop_module_t *self,
   {
     _transform_lab_to_rgb_matrix(image_in, image_out, width, height, profile_info);
   }
+  else if(cst_from == IOP_CS_RGB && cst_to == IOP_CS_RAW)
+  {
+    *converted_cst = cst_from;
+  }
   else
   {
     *converted_cst = cst_from;
@@ -1186,6 +1190,11 @@ void dt_ioppr_transform_image_colorspace
     *converted_cst = cst_from;
     return;
   }
+  if(cst_from == IOP_CS_RGB && cst_to == IOP_CS_RAW)
+  {
+    *converted_cst = cst_from;
+    return ;
+  }
 
   dt_times_t start_time = { 0 };
   dt_get_perf_times(&start_time);
@@ -1453,6 +1462,11 @@ gboolean dt_ioppr_transform_image_colorspace_cl
     *converted_cst = cst_from;
     return TRUE;
   }
+  if(cst_from == IOP_CS_RGB && cst_to == IOP_CS_RAW)
+  {
+    *converted_cst = cst_from;
+    return TRUE;
+  }
 
   const size_t ch = 4;
   float *src_buffer = NULL;