Fix conversions between GRAYA8<->GRAYA16
The color conversion system had a bug, which caused the conversion to happen via ALPHA8 color space (GRAYA8<->ALPHA16<->GRAYA16). That basically dropped the alpha channel of the GRAYA color space. It was caused by the presence of custom connections in the alpha color space.
The patch changes the way how connection path is searched. It implements NodeCapabilities structure, which tracks capabilities of the nodes used in the connection path.