libobs-opengl: Reject external-only modifiers

[asahilina]

Description

These modifiers cannot be used with the standard texture sampling codepaths, so we need to reject them to avoid invalid GL operations.

Closes #12498

Motivation and Context

See #12498, this should force cross-GPU PipeWire capture to fall back to shm if the Nvidia proprietary driver is in use, since it apparently cannot support any DMA-BUF import formats at all using standard mechanisms.

How Has This Been Tested?

Smoke tested on an Apple system. Needs testing by the affected user.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[kkartaltepe]

Seems fine on amd and intel, not that either of them have external only modifiers.

[mahkoh]

not that either of them have external only modifiers.

They do.