there is at least one texture unload without gl context

[bloerwald]

apparently there is one during texture swapping:

opengl\context.cpp (57): opengl::`anonymous namespace'::verify_context_and_check_for_gl_errors::verify_context_and_check_for_gl_errors
opengl\context.cpp (285): opengl::context::deleteTextures
opengl\texture.cpp (20): opengl::texture::~texture
noggit\multimap_with_normalized_key.hpp (89): noggit::async_object_multimap_with_normalized_key<blp_texture>::erase
qtcore\qobject_impl.h (168): QtPrivate::QFunctorSlotObject<<lambda_1f883b93a2eca3bd8ea76fc535a6773d>,0,QtPrivate::List<>,void>::impl
Qt5Widgets: QAbstractButton::click

Kind of is #12, which would address them all at once.

[bloerwald]

Duplicate of #70.