gpui only rasterizes subpixel (ClearType) glyphs when the window's
background appearance is Opaque. tty7 never asked for Opaque: with no
backdrop material the window was always Transparent (or the WCA blur),
even at the default opacity of 1.0 where nothing behind it can show
through. Every glyph on Windows therefore fell back to grayscale
antialiasing, regardless of the system ClearType setting.
On Windows, hand the window Opaque whenever the fill is fully opaque
and the resolved backdrop is plain translucency or the WCA blur. DWM
materials (Mica / Mica Alt / Acrylic) are left alone. Lowering the
opacity switches back to Transparent at runtime as before.
Fixes#1120