GM glyph_pos correct advance placement.

Glyph advances are based on the origin, not on the glyph bounding box.
Draw the tick marks for the advances starting from where the run of
glyphs was drawn.

Change-Id: I217c10a79835ad46e0178bb77ab8e575e36704ca
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368418
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>

gm/glyph_pos.cpp

@@ -138,7 +138,7 @@ static void drawTestCase(SkCanvas* canvas,
 
         // Magenta lines are the positions for the characters.
         paint.setColor(SK_ColorMAGENTA);
-        SkScalar w = bounds.x();
+        SkScalar w = 0;
         for (size_t i = 0; i < sizeof(kText) - 1; ++i) {
             canvas->drawLine(w, 0.0f, w, 5.0f, paint);
             w += widths[i];