Map a normalised 0..1 stroke point into local pixel coordinates,
letterbox-fit inside the live content rect. The stroke's recorded
aspect ratio is used (NOT the live rect's aspect) so strokes stay
anchored to the source even when the source changes resolution
mid-share — the receiver's content rect adapts to the new aspect,
but the stroke is rendered in its original aspect, letterboxed
inside the new content rect.
Map a normalised 0..1 stroke point into local pixel coordinates, letterbox-fit inside the live content rect. The stroke's recorded aspect ratio is used (NOT the live rect's aspect) so strokes stay anchored to the source even when the source changes resolution mid-share — the receiver's content rect adapts to the new aspect, but the stroke is rendered in its original aspect, letterboxed inside the new content rect.