introduce SkGlyphRect specialized for union & intersect

SkGlyphRect is a rectangle encoding specialized for
union and intersect. It will be used for calculating
the bounding boxes of glyph runs, and clipping glyphs
for GPU.

Change-Id: Icab826b51dc2254ee4006ada84f7fc09e112a933
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/319697
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index 47dc7d3..dd4ff9b 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -255,6 +255,7 @@
   "$_tests/SkFixed15Test.cpp",
   "$_tests/SkGaussFilterTest.cpp",
   "$_tests/SkGlyphBufferTest.cpp",
+  "$_tests/SkGlyphTest.cpp",
   "$_tests/SkImageTest.cpp",
   "$_tests/SkNxTest.cpp",
   "$_tests/SkPEGTest.cpp",