Added context to JPEGTileDecoder constructor

Fixed a NPE when working with the JPEGTileDecoder due to lacking a
context.
Corrected index when reading APNG PLTE

When parsing the PLTE in a an APNG image, the color table indices should
now range from [0, length/3] for each of the R, G and B arrays - instead
of ranging from [0, length] for each array, causing array index out of
bounds exceptions.
Added ImageMetadata plane creation signature

Added createPlane(ImageMetadata, x, y, w, h) signature so the metadata
can be specified (necessary for Filters that modify image metadata
information).
ChannelSeparator constructs planes correctly

The ChannelSeparatorMetadata's ImageMetadata is now passed when
a new plane is created by the ChannelSeparator. This will
reflect the effective size C and rgb channel count.
Fixed how NativeQT bpp are populated

The ImageMetadata can bases its bits per pixels purely on pixel type.
Also modified the translator bpp calculation so that rgb and pixel type
are both derived properly.
WIP: CellImg