Only gl_available/acquire obeyed the OAK_GPU_TESTS gate, so suite tests
that fabricate a GL context and texture names still reached CGL through
delete_gl_texture (and the other unguarded entry points), whose real GL
calls segfault without a current context. Turn every public wrapper
(texture/FBO/viewport/clear/readback/version/is_current) into a no-op or
error while gated, matching the Linux/Windows stubs.