Load texture io and GPU on offthread support

[halx99]

• wgl: wglShareLists
• glx: glxCreateContext
• egl: eglCreateContext
• nsgl: sharegroup
• metal: nothing to do
• vulkan: nothing to do

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.