CRT: accept data during retrace.

[TomHarte]

The actual bug here was:

• because the CRT doesn't display data received during periods of retrace, it would refuse to allocate space for any;
• but, machines might still be trying to begin provision of data in that period, even if it's not actually populated or output until later;
• ... in particular, the internal convention for outputting a single level — e.g. a border — means providing the value once and then allowing the GPU to paint it n times, so this could cause left borders in particular not to be painted, or to be painted with the wrong colour.

The particular surfacing that led me to this discovery was #1383

Resolves #1383