Fix: use 0..255 alpha scale for Lab/LCH/LABS

[hlindset]

Some of the alpha scales were off in Image.Pixel. It looks like scRGB is the only 0..1 alpha band interpretation:

# preamble: add an alpha band to jose.png
$ vips addalpha test/support/images/jose.png /tmp/jose-alpha.png

# lab is 0..255
$ vips colourspace /tmp/jose-alpha.png /tmp/jose.lab.v lab && vips getpoint /tmp/jose.lab.v 0 0
70.435928344726562 -1.6400516033172607 2.3084163665771484 255

# lch is 0..255
$ vips colourspace /tmp/jose-alpha.png /tmp/jose.lch.v lch && vips getpoint /tmp/jose.lch.v 0 0
70.435928344726562 2.8317053318023682 125.39250183105469 255

# labs is 0..255
$ vips colourspace /tmp/jose-alpha.png /tmp/jose.labs.v labs && vips getpoint /tmp/jose.labs.v 0 0
23079 -419 590 255

# scRGB is 0..1
$ vips colourspace /tmp/jose-alpha.png /tmp/jose.scrgb.v scrgb && vips getpoint /tmp/jose.scrgb.v 0 0
0.40724030137062073 0.41788515448570251 0.39157256484031677 1

Changes

• :lab/:lch alpha's scale was 0.0..1.0, now scales to 255
• :labs alpha was scaled to 65 535, now scales to 255