M4.68: BMP export at 1 to 32 bits, Windows or OS/2, with RLE and flipped rows

document.export {format: "bmp"} writes BMP through the raster export path:
depth 1 (black and white), 4 or 8 (a palette by reduction and dither,
optionally RLE4/RLE8), 16 (5-5-5), 24 or 32 (with alpha, a version 4
header); colorModel rgb or gray; fileFormat windows or os2 (a core header);
flipRows writes top-down rows (a negative height). Impossible combinations
are refused as bad params. Export As offers it with the bmpOptions dialog,
which keeps its choices writable together; BMP moves from read-only to read
and write (tests that used .bmp as the unwritable format now use .dwg).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/render/src/encode/quantize.rs

@@ -202,7 +202,7 @@ pub fn quantize(rgba: &[u8], width: u32, height: u32, o: &PaletteOptions) -> Ind
 }
 
 /// Lightness as a grey value (Rec. 601 luma).
-fn luma([r, g, b]: [u8; 3]) -> u8 {
+pub(super) fn luma([r, g, b]: [u8; 3]) -> u8 {
     ((r as u32 * 299 + g as u32 * 587 + b as u32 * 114 + 500) / 1000) as u8
 }