Enhancement: enable SIMD convolution by default

[lovell]

Many operations in libvips already use SIMD via compile-time auto-vectorisation. The simd() switch enables additional run-time SIMD for operations involving convolution such as blur, sharpen and resize.

http://sharp.pixelplumbing.com/en/stable/api-utility/#simd

This is currently an optional setting that defaults to false.

This proposal is to default the setting to true and therefore instruct libvips to take further advantage of SIMD instructions via liborc, already included in the prebuilt binaries.

The current version of liborc is v0.4.28, which has proven itself to be stable in systems that resize millions of images per day.

Does anyone, especially those who already enable this feature under heavy load, have any reservations about making this change?

[lovell]

v0.21.0 now available with this change.