ee937bdd1b routed FC_MATRIX through the cairo font matrix so synthetic
slant reaches color glyphs. But FC_MATRIX is also how fontconfig encodes
the pixel-size fixup of fixed-size faces. Noto Color Emoji is a ~109px
bitmap strike and fontconfig hands consumers a matrix scaling it to the
requested size (factor = requested_px / strike_px). cairo_set_font_size()
already brings the strike to the requested size, so feeding that matrix
into cairo_set_font_matrix() in apply_cairo_font_size() scales it down
again by the fixup factor. At terminal cell sizes that is a large shrink,
up to ~9x for small cells (easing to 1x as the cell nears the strike), so
color emoji render as a dot; fit_cairo_glyph() only shrinks, so it never
grows them back.
Only the shear carries synthetic slant; the diagonal is the size, which
cairo_set_font_size() and fit_cairo_glyph() already handle. Apply only
the shear and fall through to cairo_set_font_size() when there is no
shear. Pure-slant matrices are unchanged. This carries only the shear to
color glyphs, so a non-uniform diagonal scale from a hand-built FC_MATRIX
is dropped on that path; the stock fixup is uniform, so dropping it is
the intended behaviour.
Add a regression test that renders a color emoji and checks it fills its
cells, skipped unless a fixed-size color font with a fontconfig fixup
matrix is present.
Fixes#10144
fbd4e5ad1f