remove assertions that glyph advance width > 0

this will make zero-width glyphs disappear when rasterized via resvg to PNG, because anything outside viewbox gets clipped (#402). Thus, running maximum_color --bitmaps with a font that has such zero-width glyphs will produce invisible empty CBDT/sbix glyphs until that issue is resolved.
However, removing this assertions allows us to support zero-width glyphs in vector color formats like OT-SVG/COLR (cf. #421)

src/nanoemoji/colr_to_svg.py

@@ -327,15 +327,9 @@ def glyph_region(ttfont: ttLib.TTFont, glyph_name: str) -> Rect:
 def _view_box_and_transform(
     ttfont: ttLib.TTFont, view_box_callback: ViewboxCallback, glyph_name: str
 ) -> Tuple[Rect, Affine2D]:
-
     view_box = view_box_callback(glyph_name)
-    assert view_box.w > 0, f"0-width viewBox for {glyph_name}?!"
-
     region = glyph_region(ttfont, glyph_name)
-    assert region.w > 0, f"0-width region for {glyph_name}?!"
-
     font_to_vbox = map_font_space_to_viewbox(view_box, region)
-
     return (view_box, font_to_vbox)
 
 

src/nanoemoji/generate_svgs_from_colr.py

@@ -37,9 +37,7 @@
 
 def _view_box(font: ttLib.TTFont, glyph_name: str) -> Rect:
     # we want a viewbox that results in no scaling when translating from font-space
-    region = glyph_region(font, glyph_name)
-    assert region.w > 0, f"0-width region for {glyph_name}"
-    return region
+    return glyph_region(font, glyph_name)
 
 
 def main(argv):