A new function is added by the translator to calculate the transform
feedback offsets in the emulation path.  This function makes the
generated code for transform feedback smaller.

Bug: angleproject:3606
Change-Id: I01460f907e20e2887cb720bddad96697fdcb0cf3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2607492
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>