CAD to PDF: one page per title-block frame in model space #535

Follow-up to #172, point 1. Layouts already become one PDF page each (#471). What's left: a drawing made only in model space, with several title-block frames side by side (one sheet per frame), still becomes a single page.

Idea from #172: find the frames in model space (closed rectangles or blocks of a standard sheet size, A4–A0 or ANSI, possibly a known title-block block name) and make a page per frame, in reading order, each at the frame's own scale.

Open questions:

• how to tell a sheet frame from any other rectangle (size against standard sheets, line weight, a title-block block inside it);
• what to do when frames overlap or nest;
• whether it should be a preset option ("One page per frame") rather than automatic.