Add N/I to globals needed list when backfacing() shadeop is used

[sfriedmapixar]

Description

Fix a bug where utilizing the backfacing() shadeop would not mark the N and I globals as needed.

During the OSL runtime optimizations, if we find a backfacing() shadeop, we now add both N and I
to the list of needed globals, as the underlying implementation of that shadeop relies on them.

Tests

This is tested by the globals-needed test. Additionally it adds the ability to filter
out.txt in a test before diff'ing by including a regular expression for the filter in the filter_re variable.
This was so that the output from the debug mode of testshade could be limited to only
the line of interest to this test.

Checklist:

• [ x] I have read the contribution guidelines.
• [ x] I have previously submitted a Contributor License Agreement.
• I have updated the documentation, if applicable.
• [ x] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [x ] My code follows the prevailing code style of this project.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ sfriedmapixar (9b97cd2, 716db32)

[sfriedmapixar]

My EasyCLA manager is out on vacation for the next week, so this may be in limbo for a bit.

[lgritz]

Its been like this for a LONG time, I don't think there's a rush.

But I like the fixes. And nice trick with the testsuite option to filter the output!

[AlexMWells]

LGTM

[sfriedmapixar]

Ok, got my EasyCLA approval through. The bleeding-edge CI failure looks like a main/master rename issue. Do I need to rebase this before you can pull it, or is it good-to-go?

[lgritz]

The bleeding edge failure was already fixed by #1463, no need to worry about that. I'll just do the merge.