Use Lua addalpha impl. only for libvips < 8.16.0
#76
| Job | Run time |
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| 2m 52s | |
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| 2m 46s | |
| 4m 30s | |
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| 4m 12s | |
| 2m 55s | |
| 4m 12s | |
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| 4m 16s | |
| 1m 9s | |
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| 3m 2s | |
| 4m 8s | |
| 47m 51s |