Cleanup: Alembic: use pointers instead of references when passing progress variables

Using pointers instead of references when passing progress variables
makes the C++ code more in line with the C code (as it doesn't transform
pointer parameters to reference parameters). Also makes it easier to
spot when a common Blender pattern is implemented incorrectly (fix will
be in the next commit).

source/blender/alembic/intern/abc_exporter.cc

@@ -263,7 +263,7 @@ void AbcExporter::getFrameSet(unsigned int nr_of_samples, std::set<double> &fram
   }
 }
 
-void AbcExporter::operator()(float &progress, bool &was_canceled)
+void AbcExporter::operator()(float *progress, bool *was_canceled)
 {
   std::string scene_name;
 
@@ -332,10 +332,10 @@ void AbcExporter::operator()(float &progress, bool &was_canceled)
   size_t i = 0;
 
   for (; begin != end; ++begin) {
-    progress = (++i / size);
+    *progress = (++i / size);
 
     if (G.is_break) {
-      was_canceled = true;
+      *was_canceled = true;
       break;
     }
 

source/blender/alembic/intern/abc_exporter.h

@@ -104,7 +104,7 @@ class AbcExporter {
   AbcExporter(Main *bmain, const char *filename, ExportSettings &settings);
   ~AbcExporter();
 
-  void operator()(float &progress, bool &was_canceled);
+  void operator()(float *progress, bool *was_canceled);
 
  protected:
   void getShutterSamples(unsigned int nr_of_samples,

source/blender/alembic/intern/alembic_capi.cc

@@ -261,7 +261,7 @@ static void export_startjob(void *customdata, short *stop, short *do_update, flo
     const int orig_frame = CFRA;
 
     data->was_canceled = false;
-    exporter(*data->progress, data->was_canceled);
+    exporter(progress, &data->was_canceled);
 
     if (CFRA != orig_frame) {
       CFRA = orig_frame;