add baseline integer lerp

Add a lerp that uses full 32-bit resolution for the multiply.
This gives the same answers as the floating point, and
demonstrates the error introduced by using the faster
16-bit multiply.

Change-Id: I90c46b350fae09f74d9c9cf2af7ef3d3d1d0f202
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/448102
Bot-Commit: Rubber Stamper <rubber-stamper@appspot.gserviceaccount.com>
1 file changed