ICC engine cannot handle PQ-shaped color spaces properly. All the
HDR data will be lost. Therefore, we should implement a custom
transformation for that.

1) IccColorSpaceEngine now has a special method supportsColorSpace(),
   which declares that "SMPTE ST 2084 PQ" should not be handled by
   the engine.

2) LcmsRGBP2020PQColorSpaceFactoryWrapper adds custom conversions
   for this special color space. Before passing it to LCMS, the
   color space will be converted into Rec. 2020 Linear. After that
   ICC color management system can handle it correctly.

3) The conversions graph looks like that:

p2020-pq-u8 <-> p2020-g10-f16
p2020-pq-u16 <-> p2020-g10-f16
p2020-pq-f16 <-> p2020-g10-f32
p2020-pq-f32 <-> p2020-g10-f32

4) There is still a problem: if one decides to convert p2020-pq-f16
   into p2020-pq-u16, then the conversion will go via p2020-g10-f32.
   This is not optimal, but it can be fixed later.