Make sure that CYMRGB is same length as MAX_CHANNELS

[will-moore]

This fixes a bug when viewing images that have no omero rendering metadata, and have more than MAX_CHANNELS.

In getDefaultColors() we have:

    // Set visible indices to CYMRGB colors. visibleIndices.length === MAX_CHANNELS from above.
    for (const [i, visibleIndex] of visibleIndices.entries()) {
      colors[visibleIndex] = CYMRGB[i];
    }

but the loop goes up to where i is MAX_CHANNELS. However, the length of CYMRGB is less than this, so when i is 6, then CYMRGB[i] is undefined.

This is fixed by slicing CYMRBG to be the same length as MAX_CHANNELS.