Remaining XRImage `convert` functionality to be implemented

[katherinekolman]

These are the conversions that still need to be added that aren't in #28:

• RGB -> YCbCr, L, P
• RGBA -> YCbCrA, LA, PA
• YCbCr -> RGB, L, P
• YCbCrA -> RGBA, LA, PA
• L -> YCbCr
• LA -> YCbCrA
• P -> YCbCr, L
• PA -> YCbCrA, LA