Fix for #7609

The output colorspace struct was created every call to convert but the structure was not initialized leading to random values. Specifically reverse_bytes_in_word was referenced in the conversion function with never being set. This lead to unpredictable behavior.

Moved the colorspace object to the class structure so it can be reused and is properly initialized. This also allows the output colorspace to be made to be configurable in the future.

Tested on a KB2040.

import displayio
cc = displayio.ColorConverter(input_colorspace=displayio.Colorspace.RGB888)
hex(cc.convert(0x00ff00))

Should return 0x7e0
Before would randomly return 0xe007