Unscaled 16bit packed RGB to planar GBR converter.

Fixes r210(rgb48) -> ffv1 (gbrp10) -> r210 roundtrip
as reported by forum user JasonCA.
Support signed j2k images via libopenjpeg.

Fixes ticket #3283.

Reviewed-by: Michael Bradshaw
Fix libopenjpeg colour range adjust for 8<bpp<16.

Fixes ticket #3284.

Reviewed-by: Michael Bradshaw