math/fixed: add I and P helper functions.
math/fixed: add I and P helper functions.

The P function is especially common in e.g. Freetype code that works
with both the integer-pixel (X,Y) coordinates used by the stdlib image
package and sub-pixel (X,Y) coordinates used by its raster package.

Change-Id: I68be8ca71ceb24f40277ecc39a0896323f6671f5
Reviewed-on: https://go-review.googlesource.com/13652
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

golang/image