Fix Imagick grayscale flattening saturated colors

[nlemoine]

GrayscaleModifier on the Imagick driver turns every saturated color into the same mid grey. All the tonal information is gone.

Repro, on a 200x200 PNG with four flat quadrants (red, green, blue, yellow), sampling each quadrant centre after ->grayscale():

driver	red	green	blue	yellow
GD	76	149	29	225
Imagick (before)	128	128	128	128
Imagick (after)	54	182	18	237

Cause: the modifier calls modulateImage(100, 0, 100), which sets HSL saturation to zero. HSL lightness of a fully saturated color is (max+min)/2 = 127.5, so any saturated hue lands on 128 no matter how bright it looks.

Fix: transformImageColorspace() to COLORSPACE_GRAY, then back to COLORSPACE_SRGB. The vips driver already does the same with Interpretation::B_W then Interpretation::SRGB.

I first tried a color matrix with Rec. 601 coefficients, it would have matched GD almost exactly. But it breaks on CMYK sources. The matrix hits the C, M and Y channels, so the tonality comes out inverted, red becomes light and yellow becomes dark. The colorspace transform handles CMYK fine.

Other things I checked: alpha and semi transparency survive, animated GIFs get every frame converted, colorspace() still reports Rgb\Colorspace. Encoding to png/jpeg/gif/webp is unchanged.

Tests: testColorChange only asserted isGrayscale(), and 128,128,128 satisfies that, so nothing caught it. I added testSaturatedColorsKeepDistinctBrightness on both drivers, using the existing blocks.png fixture. It checks that pure blue, red and green stay distinct and in the right order (blue darkest, green brightest). That ordering should hold for any luma based conversion, so the same test works for GD and Imagick. On the old Imagick code it fails with Failed asserting that 128 is less than 128.