When used with HistogramLUTItem, floating and uint16 images will by default receive a 512-entry LUT. This causes the rendered QImage to use RGBA8888 format.

This PR makes ImageItem request for a 256-entry LUT, which:

1. allows rendering to the more efficient Indexed8 format. (1 byte per pixel)
2. avoids interpolation of the bundled colormaps which are natively 256-entry.