gl: make sure we detach textures and disable texturing on all gl stages

When limited range colors was in use, we could end up adjusting range
twice since the last texture stage could be left around modifying
following render.
gui: respect limited color range for test patterns
gui: let alternating circle pattern be btb or wtw if possible

Note, wtw/btb is only possible in limited color range
Merge pull request #3992 from elupus/wtwbtw

Support WTW/BTB display in test patterns