Release 3.0.0 -- twelve pixel formats

Every colour model PGF defines is now implemented, decode and encode: bitmap,
grayscale, indexed, RGB, CMYK, L*a*b, RGB48, L*a*b48, CMYK64, RGBA, RGB12 and
RGB16. Lossy for all but bitmap and indexed, which are quality-0 only because
their samples cannot survive quantization meaningfully.

MAJOR for two behaviour changes, both defect fixes: the colour-mode byte is now
a whitelist gate, so a file with an unrecognised mode on a known geometry refuses
instead of being rendered as RGB; and the encoder writes nLevels = 0 below
min(w,h) = 10, changing output bytes for small images and fixing files libpgf
misread.

Verified against libpgf as a black box: 210/210 and 210/210 at quality 0 both
directions, 420/420 each way lossy, 120/120 and 450/450 coefficient-exact
against the reference encoder, 527 tests.

Still refused: container version 0x06 (frozen), quality 19+, grayscale at 16 and
31 bpp, ROI. The indexed palette's entry order is documented, not measured.