colrv1_postproc: add vhea and vmtx to colrv1 font

copy values from CBDT font, adapt to the different UPEM (CBDT has 2048, COLRv1 has 1024, nanoemoji's default)
Maybe we shold just make the UPEM the same for both?

colrv1_postproc.py

@@ -159,6 +159,42 @@ def _map_empty_flag_tag_to_black_flag(colr_font):
     liga_set.insert(0, liga)
 
 
+def _add_vertical_layout_tables(cbdt_font, colr_font):
+    upem_scale = colr_font["head"].unitsPerEm / cbdt_font["head"].unitsPerEm
+
+    vhea = colr_font["vhea"] = ttLib.newTable("vhea")
+    vhea.tableVersion = 0x00010000
+    vhea.ascent = round(cbdt_font["vhea"].ascent * upem_scale)
+    vhea.descent = round(cbdt_font["vhea"].descent * upem_scale)
+    vhea.lineGap = 0
+    # most of the stuff below is recalculated by the compiler, but still needs to be
+    # initialized... ¯\_(ツ)_/¯
+    vhea.advanceHeightMax = 0
+    vhea.minTopSideBearing = 0
+    vhea.minBottomSideBearing = 0
+    vhea.yMaxExtent = 0
+    vhea.caretSlopeRise = 0
+    vhea.caretSlopeRun = 0
+    vhea.caretOffset = 0
+    vhea.reserved0 = 0
+    vhea.reserved1 = 0
+    vhea.reserved2 = 0
+    vhea.reserved3 = 0
+    vhea.reserved4 = 0
+    vhea.metricDataFormat = 0
+    vhea.numberOfVMetrics = 0
+
+    # emoji font is monospaced -- except for an odd uni0000 (NULL) glyph which happens
+    # to have height=0; but colrv1 font doesn't have that anyway, so I just skip it
+    cbdt_heights = set(h for h, _ in cbdt_font["vmtx"].metrics.values() if h != 0)
+    assert len(cbdt_heights) == 1, "NotoColorEmoji CBDT font should be monospaced!"
+    height = round(cbdt_heights.pop() * upem_scale)
+    vmtx = colr_font["vmtx"] = ttLib.newTable("vmtx")
+    vmtx.metrics = {}
+    for gn in colr_font.getGlyphOrder():
+        vmtx.metrics[gn] = height, 0
+
+
 def main(argv):
     if len(argv) != 3:
         raise ValueError(
@@ -194,6 +230,8 @@ def main(argv):
 
     add_soft_light_to_flags(colr_font)
 
+    _add_vertical_layout_tables(cbdt_font, colr_font)
+
     out_file = Path("fonts/Noto-COLRv1-noflags.ttf").absolute()
     print("Writing", out_file)
     colr_font.save(out_file)