[canvaskit] Add in fillText to spreadsheet demo for comparison

The drawGlyphs implementation seems to have a 20-30% performance
edge and drawText is a few percent slower than fillText on my
machine.

Change-Id: I5b8a623bcd4fdbfea3d534d3dbaf3d6f38d37e55
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/508921
Reviewed-by: Ben Wagner <bungeman@google.com>
2 files changed