Render the MTEXT background (never drawn before — the flags were only shown
read-only in the panel) and make it editable.
- Text arm emits an opaque fill_tris rect behind the glyphs: flag 0x01 uses
the entity background colour, 0x02 uses the canvas colour (a wipeout-style
mask). Box = glyph bounds padded by (background_scale - 1) × text height.
- Text frame (flag 0x10) draws a closed rectangle in the text colour around
the same box.
- Threaded `bg_color` into `tessellate()` (+ all callers and the
MultiLeaderTess trait) so the mask can use the drawing-window colour.
- Properties: "Background mask" is a Off / Fill / Mask dropdown, "Background
color" is its own colour picker (independent of the entity colour — new
PropBgColor* messages; picking a colour switches to Fill mode), and "Text
frame" is a checkbox toggle. Edits re-tessellate so the change shows live.
Renders via the face3d fill path, so the fill shows with FILLMODE on (same as
MLEADER background fill); the frame is a stroke and always shows.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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