Turn on Meson release build optimizations.

[adworacz]

Just like my other PR, nice performance boost with using the -O3 defaults of meson in release mode:

Benchmark script

import vapoursynth as vs
core = vs.core

clip = core.ffms2.Source('./source.mkv')

clip = clip.vivtc.VFM(1)

clip.set_output()

Results:

• Raw clip: 736fps
• Old code: 280 fps
• New code: 430 fps

I only benched with VFM as I have a progressive input, but I'm sure VDecimate has some gains as well. I can rerun benchmarks to include it if desired.