pcx: multiple fixes (2)

- 1-bit writer: checks where is black and use NOT operator only if needed
- Fix images with witdh == 65536(*)
- Checks result of disk writes and reads on all formats

(*) PCX formats support images with with of 65536 but only if the header field bytesPerLine is valid (no overflow). This means that the width 65536 is supported on 1bpp images only.
The previous version of the plugins wrote an image with width of 65536px in the wrong way and it was unable to read it (wrong image returned). I verified that Photoshop and Gimp weren't able to read the image either.