drawOver
Draws this overlay onto RGBA pixels, in place.
For burning subtitles into a screenshot. The engine's own renderers composite on the GPU and never come here; this is the path for a caller holding a CapturedFrame that wants one image with the text on it.
The cue alpha contract is premultiplied end to end, so the blend is dst = src + dst * (1 - a) with no divide anywhere. Premultiplying again here would render white text grey.
Pixels outside the overlay are untouched, so an overlay with nothing in it costs nothing.
Parameters
Throws
when rgba is not that size, or when the overlay was laid out for a different viewport: drawing it anyway would put the text in the wrong place.