LibGfx: Add Emoji::emoji_for_code_point_iterator(Utf8CodePointIterator&)

In the common case of text rendering rather than getting the emoji
bitmap for a fixed number of code points, we don't know how many code
points make one emoji. As far as I can tell, the longest ones are up to
ten code points, so we try to consume all of them and do a lookup during
each iteration, and return the emoji for the longest chain of code
points. Quite basic and definitely room for improvement, but it works!

Userland/Libraries/LibGfx/Emoji.cpp

@@ -8,6 +8,7 @@
 #include <AK/HashMap.h>
 #include <AK/Span.h>
 #include <AK/String.h>
+#include <AK/Utf8View.h>
 #include <LibGfx/Bitmap.h>
 #include <LibGfx/Emoji.h>
 
@@ -41,4 +42,48 @@ Bitmap const* Emoji::emoji_for_code_points(Span<u32> const& code_points)
     return bitmap.ptr();
 }
 
+Bitmap const* Emoji::emoji_for_code_point_iterator(Utf8CodePointIterator& it)
+{
+    // NOTE: I'm sure this could be more efficient, e.g. by checking if each code point falls
+    // into a certain range in the loop below (emojis, modifiers, variation selectors, ZWJ),
+    // and bailing out early if not. Current worst case is 10 file lookups for any sequence of
+    // code points (if the first glyph isn't part of the font in regular text rendering).
+
+    constexpr size_t max_emoji_code_point_sequence_length = 10;
+
+    Vector<u32, max_emoji_code_point_sequence_length> code_points;
+
+    struct EmojiAndCodePoints {
+        Bitmap const* emoji;
+        Span<u32> code_points;
+    };
+    Vector<EmojiAndCodePoints, max_emoji_code_point_sequence_length> possible_emojis;
+
+    // Determine all existing emojis for the longest possible ZWJ emoji sequence,
+    // or until we run out of code points in the iterator.
+    for (size_t i = 0; i < max_emoji_code_point_sequence_length; ++i) {
+        auto code_point = it.peek(i);
+        if (!code_point.has_value())
+            break;
+        code_points.append(*code_point);
+        if (auto const* emoji = emoji_for_code_points(code_points))
+            possible_emojis.empend(emoji, code_points);
+    }
+
+    if (possible_emojis.is_empty())
+        return nullptr;
+
+    // If we found one or more matches, return the longest, i.e. last. For example:
+    // U+1F3F3 - white flag
+    // U+1F3F3 U+FE0F U+200D U+1F308 - rainbow flag
+    auto& [emoji, emoji_code_points] = possible_emojis.last();
+
+    // Advance the iterator, so it's on the last code point of our found emoji and
+    // whoever is iterating will advance to the next new code point.
+    for (size_t i = 0; i < emoji_code_points.size() - 1; ++i)
+        ++it;
+
+    return emoji;
+}
+
 }