Images are allocated without being initialized, so a reader must write
every pixel of every line. The PCX reader did not always:
- readImage2() and readImage4v2() decoded width / 4 and width / 2 bytes,
so the last pixels of lines whose width is not a multiple of 4 (2 bits)
or 2 (4 bits) were not set. They now decode every pixel.
- With a BytesPerLine smaller than a line of pixels, readImage1(),
readImage8() and readImage24() left the end of each line unset: such
headers are now refused by isSupported(), which replaces the check
readImage4() had.
Added the read tests indexed2_w13.pcx and indexed4_w13.pcx, which fail
without this change.
