Use unorm shorts for texture coordinates when rendering text.

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.
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Committed: https://skia.googlesource.com/skia/+/e507ff0460f4f878214b9454fb5b9ab8d64d8063

Review URL: https://codereview.chromium.org/1713693002