Auto convert 1- and 4-bit images to 8-bit when processing

[dataflowg]

For indexed images, processing routines only work on 8-bit. Add functions to auto convert 1- and 4-bit to 8-bit when processing, and back to 1-bit or 4-bit when complete. Similar to the flatten / unflatten pixmap VIs, but using a 1D U8 array as input / output.