Paints a colour through a stencil mask in the unit square of user space: where the mask paints, the colour is painted.
void FillImageMask(DisplayImageMask mask, in DisplayColor color)
The image mask, the same instance for every paint of one mask XObject in a list; it maps to the unit square like an image (§8.9.4).
The nonstroking colour, unconverted.
ISO 32000-2 §8.9.6.2 (stencil masking, ImageMask true): the mask's samples say where the current fill colour is painted
and where the backdrop shows through; §8.9.5.3 (Interpolate) applies when it is enlarged. A stencil mask in a Pattern colour
is not in the list yet.