vips: add a vips__gaussblur method

handle both < 7.41 and higher.
Prior 7.41, vips_gaussblur took only a int param, the radius.
The radius was then divided by 2.0 (min_ampl) in vips_gaussblur.
After, you can now parameter the min_ampl and radius became sigma (and
passed from an integer to a double).

options.go

@@ -91,6 +91,11 @@ type Watermark struct {
 	Background  Color
 }
 
+type GaussianBlur struct {
+	Sigma   float64
+	MinAmpl float64
+}
+
 type Options struct {
 	Height         int
 	Width          int
@@ -117,4 +122,5 @@ type Options struct {
 	Type           ImageType
 	Interpolator   Interpolator
 	Interpretation Interpretation
+	GaussianBlur   GaussianBlur
 }

vips.go

@@ -432,3 +432,14 @@ func boolToInt(b bool) int {
 	}
 	return 0
 }
+
+func vipsGaussianBlur(image *C.VipsImage, o GaussianBlur) (*C.VipsImage, error) {
+	var out *C.VipsImage
+	defer C.g_object_unref(C.gpointer(image))
+
+	err := C.vips__gaussblur(image, &out, C.double(o.Sigma), C.double(o.MinAmpl))
+	if err != 0 {
+		return nil, catchVipsError()
+	}
+	return out, nil
+}

vips.h

@@ -16,6 +16,9 @@
  */
 
 #if (VIPS_MAJOR_VERSION == 7 && VIPS_MINOR_VERSION < 41)
+/* we need math.h for ceil() in vips__gaussblur */
+#include <math.h>
+
 #define VIPS_ANGLE_D0 VIPS_ANGLE_0
 #define VIPS_ANGLE_D90 VIPS_ANGLE_90
 #define VIPS_ANGLE_D180 VIPS_ANGLE_180
@@ -321,3 +324,12 @@ vips_watermark(VipsImage *in, VipsImage **out, WatermarkTextOptions *to, Waterma
 	g_object_unref(base);
 	return 0;
 }
+
+int
+vips__gaussblur(VipsImage *in, VipsImage **out, double sigma, double min_ampl) {
+#if (VIPS_MAJOR_VERSION == 7 && VIPS_MINOR_VERSION < 41)
+	return vips_gaussblur(in, out, ceil(sigma), NULL);
+#else
+	return vips_gaussblur(in, out, sigma, NULL, "min_ampl", min_ampl);
+#endif
+}