fix(render): give degenerate segments area, pin font bit order, clarify vertex_count

tests/test_primitives.cpp

@@ -151,6 +151,40 @@ PLOT_TEST(bitmap_font_decodes_si_units_and_falls_back_visibly) {
   PLOT_CHECK(any_ink);
 }
 
+// The packed atlas must be left-to-right in absolute terms: `(row >> x) & 1` is
+// also how every other expectation here is derived, so a table transcribed with
+// the opposite in-row bit order would keep those tests green while drawing
+// mirrored text. 'A' cannot catch that (it is
+// symmetric about its vertical axis); 'L', 'b' and '/' can.
+PLOT_TEST(bitmap_font_bit_order_runs_left_to_right) {
+  using plot::render::BitmapFont;
+  const plot::u8* atlas = BitmapFont::atlas_image();
+  auto ink = [atlas](char c, plot::u32 col, plot::u32 row) {
+    const plot::u32 slot = static_cast<plot::u32>(c) - BitmapFont::kFirstChar;
+    const plot::Vec2 origin = BitmapFont::atlas_origin(slot);
+    const auto x = static_cast<std::size_t>(origin.x) + col;
+    const auto y = static_cast<std::size_t>(origin.y) + row;
+    return atlas[y * BitmapFont::kAtlasWidth + x] != 0u;
+  };
+  // 'L': stem and top serif on the left, the foot running right along the
+  // bottom row and stopping one texel short of the right edge.
+  PLOT_CHECK(ink('L', 0u, 0u));
+  PLOT_CHECK(!ink('L', 7u, 0u));
+  PLOT_CHECK(ink('L', 1u, 3u));
+  PLOT_CHECK(!ink('L', 6u, 3u));
+  PLOT_CHECK(ink('L', 6u, 6u));
+  PLOT_CHECK(!ink('L', 7u, 6u));
+  // 'b': the stem is the leftmost column of the upper rows.
+  PLOT_CHECK(ink('b', 0u, 0u));
+  PLOT_CHECK(!ink('b', 7u, 0u));
+  // '/': starts at the right of the top row and ends at the left of the last
+  // one — the opposite of '\'.
+  PLOT_CHECK(ink('/', 6u, 0u));
+  PLOT_CHECK(!ink('/', 0u, 0u));
+  PLOT_CHECK(ink('/', 0u, 6u));
+  PLOT_CHECK(!ink('/', 6u, 6u));
+}
+
 PLOT_TEST(bitmap_font_atlas_is_uploadable) {
   using plot::render::BitmapFont;
   PLOT_CHECK(BitmapFont::kAtlasWidth == BitmapFont::kAtlasCols * BitmapFont::kGlyphPx);