Refine GifExportParams (#417)

* Refine GifExportParams

* Change gifDither range back to 0~10

* Add StripMetadata back

* Add StripMetadata back

Author: n0vad3v

vips/foreign.c

@@ -319,7 +319,8 @@ int set_tiffsave_options(VipsOperation *operation, SaveParams *params) {
 
 // https://libvips.github.io/libvips/API/current/VipsForeignSave.html#vips-magicksave-buffer
 int set_magicksave_options(VipsOperation *operation, SaveParams *params) {
-  int ret = vips_object_set(VIPS_OBJECT(operation), "format", "GIF", NULL);
+  int ret = vips_object_set(VIPS_OBJECT(operation), "format", "GIF", "bitdepth", params->gifBitdepth, NULL);
+
   if (!ret && params->quality) {
     ret = vips_object_set(VIPS_OBJECT(operation), "quality", params->quality,
                           NULL);
@@ -331,7 +332,7 @@ int set_magicksave_options(VipsOperation *operation, SaveParams *params) {
 int set_gifsave_options(VipsOperation *operation, SaveParams *params) {
   int ret = 0;
   // See for argument values: https://www.libvips.org/API/current/VipsForeignSave.html#vips-gifsave
-  if (params->gifDither > 0.0 && params->gifDither <= 1.0) {
+  if (params->gifDither > 0.0 && params->gifDither <= 10) {
     ret = vips_object_set(VIPS_OBJECT(operation), "dither", params->gifDither, NULL);
   }
   if (params->gifEffort >= 1 && params->gifEffort <= 10) {

vips/image.go

@@ -314,6 +314,10 @@ func NewTiffExportParams() *TiffExportParams {
 	}
 }
 
+// GifExportParams are options when exporting a GIF to file or buffer
+// Please note that if vips version is above 8.12, then `vips_gifsave_buffer` is used, and only `Dither`, `Effort`, `Bitdepth` is used.
+// If vips version is below 8.12, then `vips_magicksave_buffer` is used, and only `Bitdepth`, `Quality` is used.
+// StripMetadata does nothing to Gif images.
 type GifExportParams struct {
 	StripMetadata bool
 	Quality       int