The GRAPHEMES_BUF data kind previously required a double pointer
(pointer to a uint32_t*) because the OutType was [*]u32, making the
typed out parameter *[*]u32. Change OutType to u32 so that callers
pass a plain uint32_t* buffer directly, which is the natural C
calling convention. The implementation casts the out pointer to
[*]u32 internally to write into the buffer.
The STYLE data kind read directly from the render state style array
without checking whether the cell actually had non-default styling.
The style data is undefined for unstyled cells, so this caused a
panic on a corrupt enum value when the caller read the style of an
unstyled cell. Now check cell.hasStyling() first and return the
default style for unstyled cells.
Expand the c-vt-render example to exercise dirty tracking, color
retrieval, cursor state, row/cell iteration with style resolution,
and dirty state reset. Break the example into six doxygen snippet
regions and reference them from render.h.
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