Cycles: Free unused image buffers when rendering with locked interface

It is still possible to free a bit more memory by detecting buildin images
which are not used by shaders, but that's not going to improve memory usage
that much to bother about this now.

Such change brings peak memory usage from 4.1GB to 3.4GB when rendering
01_01_01_D layout scene from the Gooseberry project. Mainly because of
freeing memory used by rather huge environment map in the viewport.

Reviewers: campbellbarton, juicyfruit

Subscribers: eyecandy

Differential Revision: https://developer.blender.org/D1215

source/blender/makesrna/intern/rna_image_api.c

@@ -288,6 +288,11 @@ static void rna_Image_filepath_from_user(Image *image, ImageUser *image_user, ch
 	BKE_image_user_file_path(image_user, image, filepath);
 }
 
+static void rna_Image_buffers_free(Image *image)
+{
+	BKE_image_free_buffers(image);
+}
+
 #else
 
 void RNA_api_image(StructRNA *srna)
@@ -374,6 +379,9 @@ void RNA_api_image(StructRNA *srna)
 	RNA_def_property_flag(parm, PROP_THICK_WRAP);  /* needed for string return value */
 	RNA_def_function_output(func, parm);
 
+	func = RNA_def_function(srna, "buffers_free", "rna_Image_buffers_free");
+	RNA_def_function_ui_description(func, "Free the image buffers from memory");
+
 	/* TODO, pack/unpack, maybe should be generic functions? */
 }