fix(mtext): treat the 0x02 background-fill flag as a mask over 0x01

MTEXT background fill flag 0x02 means "use the drawing-window colour" — a mask
that erases geometry behind the text — while 0x01 means "paint background_color".
The fill-colour pick tested 0x01 first, so text flagged 0x03 (both bits, e.g. a
dimension whose DIMTFILL is the drawing background) painted the stored
background_color (a stray grey box) instead of masking. AutoCAD shows no colour
there.

Test 0x02 first: when set, fill with the drawing-window colour (invisible on a
matching canvas) regardless of 0x01; a plain 0x01 fill still paints its colour.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/convert/tessellate.rs

@@ -638,10 +638,17 @@ pub fn tessellate(
                                 );
                                 // Fill / mask — two triangles behind the glyphs.
                                 if has_fill {
-                                    let fill_color = if m.background_fill_flags & 0x01 != 0 {
-                                        color_or_inherit(&m.background_color, bg_color)
-                                    } else {
+                                    // 0x02 (use the drawing-window colour) is a
+                                    // MASK — it wins when set, even alongside
+                                    // 0x01, so text flagged "drawing background"
+                                    // erases what's behind it (a dark box on a
+                                    // dark canvas = no visible colour), instead
+                                    // of painting the stored background_color. A
+                                    // plain 0x01 fill paints that colour.
+                                    let fill_color = if m.background_fill_flags & 0x02 != 0 {
                                         bg_color
+                                    } else {
+                                        color_or_inherit(&m.background_color, bg_color)
                                     };
                                     let corners = [[l, b], [r, b], [r, t], [l, t]];
                                     let mut ft = Vec::with_capacity(6);