Expose optional minAmpl parameter of blur operation

[marcosc90]

following #4168, this PR adds minAmpl option to .blur(), completing the exposure of all options of vips_gaussblur().

[lovell]

Thank you for the PR Marcos.

I usually prefer for the parameter names in sharp to match libvips for consistency as you've done here but I feel "ampl" might be better expanded in the external API to "amplitude" to remove any ambiguity, perhaps as minAmplitude.

It might also be good to include the libvips description of this property in the JSDocs, something like "A smaller value will generate a larger, more accurate mask".

https://github.com/libvips/libvips/blob/81b6b783f09fd7acd0e0ede1a28fef906ce810e9/libvips/convolution/gaussblur.c#L186

[marcosc90]

I usually prefer for the parameter names in sharp to match libvips for consistency as you've done here but I feel "ampl" might be better expanded in the external API to "amplitude" to remove any ambiguity, perhaps as minAmplitude.

I had the same thought, I preferred minAmplitude but went with minAmpl to mimic libvips. I'll push the changes in a bit.

[marcosc90]

@lovell I left internal API with minAmpl, let me know if you want this changed as well.

[lovell]

Thank you very much!