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
 }