refactor(render): TextBackend owns its atlas, and staged glyphs carry a rotation basis

src/render/text.cpp

@@ -0,0 +1,48 @@
+#include <plot/render/text.hpp>
+
+namespace plot::render {
+
+// The decoder lives with the interface, not with a font, because it is what keeps
+// "what a string is worth" (`TextBackend::measure`) and "what a string draws"
+// (`TextBatch::add_text`) the same thing: both walk a label with this function, so
+// neither can disagree with the other about where a codepoint starts or how many
+// bytes a malformed sequence costs.
+u32 next_codepoint(const char*& p) noexcept {
+  const auto b0 = static_cast<u32>(static_cast<unsigned char>(p[0]));
+  if (b0 < 0x80u) {
+    ++p;
+    return b0;
+  }
+  u32 codepoint = 0;
+  u32 extra = 0;
+  if ((b0 & 0xE0u) == 0xC0u) {
+    codepoint = b0 & 0x1Fu;
+    extra = 1u;
+  } else if ((b0 & 0xF0u) == 0xE0u) {
+    codepoint = b0 & 0x0Fu;
+    extra = 2u;
+  } else if ((b0 & 0xF8u) == 0xF0u) {
+    codepoint = b0 & 0x07u;
+    extra = 3u;
+  } else {
+    ++p;
+    return kInvalidCodepoint;
+  }
+  for (u32 i = 1; i <= extra; ++i) {
+    const auto b = static_cast<u32>(static_cast<unsigned char>(p[i]));
+    if ((b & 0xC0u) != 0x80u) {  // truncated: stop before the terminator
+      ++p;
+      return kInvalidCodepoint;
+    }
+    codepoint = (codepoint << 6) | (b & 0x3Fu);
+  }
+  p += extra + 1u;
+  constexpr u32 kMinForLength[4] = {0u, 0x80u, 0x800u, 0x10000u};
+  if (codepoint < kMinForLength[extra] || codepoint > 0x10FFFFu ||
+      (codepoint >= 0xD800u && codepoint <= 0xDFFFu)) {
+    return kInvalidCodepoint;  // overlong, out of range or a surrogate
+  }
+  return codepoint;
+}
+
+}  // namespace plot::render