Use unorm shorts for texture coordinates when rendering text.

There are a couple of reasons for this:
- Vulkan does not guarantee conversions from integral vertex attributes
  to floating point shader variables
- This may be faster and more precise on some platforms, as it avoids
  the aforementioned conversion and changes a multiply by a very small
  value to a multiply by a medium-sized value.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1713693002
TBR=bsalomon@google.com

Review URL: https://codereview.chromium.org/1713693002
9 files changed