fix(fonttools): keep natural glyph proportions in the ASCII atlas

build_ascii_atlas() used to scale every glyph to fill its cw x ch cell,
so uppercase and lowercase had the same height and narrow shapes such as
i/l/o became solid blobs.

Use one uniform scale ((ch - 2) / units_per_em) for the whole font, keep
the real cap/x-height/descender ratios, and only squeeze glyphs whose ink
is wider than cw - 1. Center by ink width and place the baseline at ch - 4.

vpsdk/fonttools/fonttools.vp

@@ -522,10 +522,14 @@ def build_ascii_atlas(font: Font*, cw: int, ch: int) -> uint8*:
                 bw: int = xmax - xmin
                 bh: int = ymax - ymin
                 if bw > 0 and bh > 0:
-                    a: float = float(cw - 2) / float(bw)
-                    d: float = float(ch - 4) / float(bh)
-                    tx: float = 1.0 - float(xmin) * a
-                    ty: float = -2.0 - float(ymax) * d
+                    # Natural metrics: one uniform scale for the whole font, so
+                    # cap/x-height and ascender/descender keep their real
+                    # proportions. Only glyphs wider than the cell are squeezed.
+                    scale: float = float(ch - 2) / float(font.units_per_em)
+                    a: float = scale
+                    if float(bw) * scale > float(cw - 1):
+                        a = float(cw - 1) / float(bw)
+                    tx: float = (float(cw) - float(bw) * a) / 2.0 - float(xmin) * a
                     string.memset(&_ascii_tmp, 0, uint64(cw * ch * 4))
                     global _edge_count, _min_x, _max_x, _min_y, _max_y
                     _edge_count = 0
@@ -533,7 +537,7 @@ def build_ascii_atlas(font: Font*, cw: int, ch: int) -> uint8*:
                     _max_x = -1.0e30
                     _min_y = 1.0e30
                     _max_y = -1.0e30
-                    _add_glyph(font, gid, a, 0.0, 0.0, d, tx, ty, 0.0, 0.0, 1.0, 0)
+                    _add_glyph(font, gid, a, 0.0, 0.0, scale, tx, 0.0, 0.0, float(ch - 4), 1.0, 0)
                     _fill_edges(&_ascii_tmp[0], cw, ch, uint32(0x00FFFFFF))
                     py: int = 0
                     while py < ch: