Move GrTextureStripAtlas to its own file

This is a purely mechanical change and intended to make the diff of
https://skia-review.googlesource.com/c/skia/+/144305(Update GrTextureStripAtlas for DDLs) comprehensible.

Change-Id: I85fe54dd26aef6c86536702c2e19bd3226d44ba2
Reviewed-on: https://skia-review.googlesource.com/144788
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed