[Impeller] avoid reading font while parsing sktextblob. (#55442)

Use a faster approximation for finding out axis alignment that skips opening the font file, which is slow!

![image](https://github.com/user-attachments/assets/ae840597-b63b-4e46-b857-ac5f09e09ca8)

Author: jonahwilliams

impeller/typographer/backends/skia/text_frame_skia.cc

@@ -19,25 +19,6 @@
 
 namespace impeller {
 
-/// @brief Convert a Skia axis alignment into an Impeller alignment.
-///
-///        This does not include a case for AxisAlignment::kNone, that should
-///        be used if SkFont::isSubpixel is false.
-static AxisAlignment ToAxisAligment(SkAxisAlignment aligment) {
-  switch (aligment) {
-    case SkAxisAlignment::kNone:
-      // Skia calls this case none, meaning alignment in both X and Y.
-      // Impeller will call it "all" since that is less confusing. "none"
-      // is reserved for no subpixel alignment.
-      return AxisAlignment::kAll;
-    case SkAxisAlignment::kX:
-      return AxisAlignment::kX;
-    case SkAxisAlignment::kY:
-      return AxisAlignment::kY;
-  }
-  FML_UNREACHABLE();
-}
-
 static Font ToFont(const SkTextBlobRunIterator& run, AxisAlignment alignment) {
   auto& font = run.font();
   auto typeface = std::make_shared<TypefaceSkia>(font.refTypeface());
@@ -63,35 +44,35 @@ std::shared_ptr<TextFrame> MakeTextFrameFromTextBlobSkia(
   bool has_color = false;
   std::vector<TextRun> runs;
   for (SkTextBlobRunIterator run(blob.get()); !run.done(); run.next()) {
-    // TODO(112005): Ask Skia for a public API to look this up. This is using a
-    // private API today.
     SkStrikeSpec strikeSpec = SkStrikeSpec::MakeWithNoDevice(run.font());
     SkBulkGlyphMetricsAndPaths paths{strikeSpec};
+    SkSpan<const SkGlyph*> glyphs =
+        paths.glyphs(SkSpan(run.glyphs(), run.glyphCount()));
+
+    for (const auto& glyph : glyphs) {
+      has_color |= glyph->isColor();
+    }
+
     AxisAlignment alignment = AxisAlignment::kNone;
-    if (run.font().isSubpixel()) {
-      alignment = ToAxisAligment(
-          strikeSpec.createScalerContext()->computeAxisAlignmentForHText());
+    if (run.font().isSubpixel() && run.font().isBaselineSnap() && !has_color) {
+      alignment = AxisAlignment::kX;
     }
 
-    const auto glyph_count = run.glyphCount();
-    const auto* glyphs = run.glyphs();
     switch (run.positioning()) {
       case SkTextBlobRunIterator::kFull_Positioning: {
         std::vector<TextRun::GlyphPosition> positions;
-        positions.reserve(glyph_count);
-        for (auto i = 0u; i < glyph_count; i++) {
+        positions.reserve(run.glyphCount());
+        for (auto i = 0u; i < run.glyphCount(); i++) {
           // kFull_Positioning has two scalars per glyph.
           const SkPoint* glyph_points = run.points();
           const SkPoint* point = glyph_points + i;
-          Glyph::Type type = paths.glyph(glyphs[i])->isColor()
-                                 ? Glyph::Type::kBitmap
-                                 : Glyph::Type::kPath;
-          has_color |= type == Glyph::Type::kBitmap;
-          positions.emplace_back(
-              TextRun::GlyphPosition{Glyph{glyphs[i], type}, Point{
-                                                                 point->x(),
-                                                                 point->y(),
-                                                             }});
+          Glyph::Type type =
+              glyphs[i]->isColor() ? Glyph::Type::kBitmap : Glyph::Type::kPath;
+          positions.emplace_back(TextRun::GlyphPosition{
+              Glyph{glyphs[i]->getGlyphID(), type}, Point{
+                                                        point->x(),
+                                                        point->y(),
+                                                    }});
         }
         TextRun text_run(ToFont(run, alignment), positions);
         runs.emplace_back(text_run);