Move vertex buffer setup out of drawPackedGlyph().

This gets a very welcome ~10% speedup on my Mac.

Committed: https://skia.googlesource.com/skia/+/1d38619389cc1100bd516b9acbf17b7bd42d33ce

R=bsalomon@google.com

Author: jvanverth@google.com

Review URL: https://codereview.chromium.org/414573002
diff --git a/src/gpu/GrTextStrike.h b/src/gpu/GrTextStrike.h
index 89b0542..a5bb6ae 100644
--- a/src/gpu/GrTextStrike.h
+++ b/src/gpu/GrTextStrike.h
@@ -34,6 +34,7 @@
     const GrFontDescKey* getFontScalerKey() const { return fFontScalerKey; }
     GrFontCache* getFontCache() const { return fFontCache; }
     GrMaskFormat getMaskFormat() const { return fMaskFormat; }
+    GrTexture*   getTexture() const { return fAtlas->getTexture(); }
 
     inline GrGlyph* getGlyph(GrGlyph::PackedID, GrFontScaler*);
     bool addGlyphToAtlas(GrGlyph*, GrFontScaler*);