This encapsulates most of the logic for a Program into a helper class.
Now we can store multiple instances of the Program's back-end to
implement different shader behaviour at draw time.

This will be useful for shader patching for OpenGL line segment raster.

Bug: angleproject:2598
Change-Id: I800a737088574e28f3a4ec23b91c0cb2647e4e12
Reviewed-on: https://chromium-review.googlesource.com/1127302Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>