vt: fix render state cell style and graphemes_buf APIs

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.

include/ghostty/vt/render.h

@@ -58,10 +58,26 @@ extern "C" {
  * reset the row-level dirty flags. So, the caller of the render state API must
  * be careful to manage both layers of dirty state correctly. 
  *
- * ## Example
+ * ## Examples
  *
+ * ### Creating and updating render state
  * @snippet c-vt-render/src/main.c render-state-update
  *
+ * ### Checking dirty state
+ * @snippet c-vt-render/src/main.c render-dirty-check
+ *
+ * ### Reading colors
+ * @snippet c-vt-render/src/main.c render-colors
+ *
+ * ### Reading cursor state
+ * @snippet c-vt-render/src/main.c render-cursor
+ *
+ * ### Iterating rows and cells
+ * @snippet c-vt-render/src/main.c render-row-iterate
+ *
+ * ### Resetting dirty state after rendering
+ * @snippet c-vt-render/src/main.c render-dirty-reset
+ *
  * @{
  */