LibGfx: Decouple glyph positions calculation from draw_text_run()

This change separates a part of the `draw_text_run()` function, which
is responsible for calculating the positions for glyphs that need to be
painted, into a separate function called `get_glyph_run()`.

It is a part of the preparation for text run painting using OpenGL,
where we can't immediately blit glyph bitmaps but instead need to
prepare a sequence of quads for them in advance.

Userland/Libraries/LibGfx/TextLayout.cpp

@@ -6,6 +6,10 @@
  */
 
 #include "TextLayout.h"
+#include "Font/Emoji.h"
+#include <AK/Debug.h>
+#include <LibUnicode/CharacterTypes.h>
+#include <LibUnicode/Emoji.h>
 
 namespace Gfx {
 
@@ -192,4 +196,74 @@ DeprecatedString TextLayout::elide_text_from_right(Utf8View text) const
     return text.as_string();
 }
 
+DrawGlyphOrEmoji prepare_draw_glyph_or_emoji(FloatPoint point, Utf8CodePointIterator& it, Font const& font)
+{
+    u32 code_point = *it;
+    auto next_code_point = it.peek(1);
+
+    ScopeGuard consume_variation_selector = [&, initial_it = it] {
+        static auto const variation_selector = Unicode::property_from_string("Variation_Selector"sv);
+        if (!variation_selector.has_value())
+            return;
+
+        // If we advanced the iterator to consume an emoji sequence, don't look for another variation selector.
+        if (initial_it != it)
+            return;
+
+        // Otherwise, discard one code point if it's a variation selector.
+        if (next_code_point.has_value() && Unicode::code_point_has_property(*next_code_point, *variation_selector))
+            ++it;
+    };
+
+    // NOTE: We don't check for emoji
+    auto font_contains_glyph = font.contains_glyph(code_point);
+    auto check_for_emoji = !font.has_color_bitmaps() && Unicode::could_be_start_of_emoji_sequence(it, font_contains_glyph ? Unicode::SequenceType::EmojiPresentation : Unicode::SequenceType::Any);
+
+    // If the font contains the glyph, and we know it's not the start of an emoji, draw a text glyph.
+    if (font_contains_glyph && !check_for_emoji) {
+        return DrawGlyph {
+            .position = point,
+            .code_point = code_point,
+            .font = &font,
+        };
+    }
+
+    // If we didn't find a text glyph, or have an emoji variation selector or regional indicator, try to draw an emoji glyph.
+    if (auto const* emoji = Emoji::emoji_for_code_point_iterator(it)) {
+        return DrawEmoji {
+            .position = point.to_type<int>(),
+            .emoji = emoji,
+            .font = &font,
+        };
+    }
+
+    // If that failed, but we have a text glyph fallback, draw that.
+    if (font_contains_glyph) {
+        return DrawGlyph {
+            .position = point,
+            .code_point = code_point,
+            .font = &font,
+        };
+    }
+
+    // No suitable glyph found, draw a replacement character.
+    dbgln_if(EMOJI_DEBUG, "Failed to find a glyph or emoji for code_point {}", code_point);
+    return DrawGlyph {
+        .position = point,
+        .code_point = 0xFFFD,
+        .font = &font,
+    };
+}
+
+Vector<DrawGlyphOrEmoji> get_glyph_run(FloatPoint baseline_start, Utf8View const& string, Font const& font, IncludeLeftBearing include_left_bearing)
+{
+    Vector<DrawGlyphOrEmoji> glyphs_or_emojis;
+    for_each_glyph_position(
+        baseline_start, string, font, [&](auto glyph_or_emoji) {
+            glyphs_or_emojis.append(glyph_or_emoji);
+        },
+        include_left_bearing);
+    return glyphs_or_emojis;
+}
+
 }