Add RenderTarget:Clone

[yogwoggf]

Fixes #82

PR Type (tick all that are applicable)

• [ ] Bug Fix
• [ ] New Feature (doesn't break Module <-> Lua compatibility)
• [x] New Feature (breaks Module <-> Lua compatibility)
• [x] Documentation Update Required

Tested Targets (only applicable for changes to the binary module, delete otherwise)

• [x] windows-x64-release
• [ ] windows-x86-release

Checklist

• [x] I have read and understand the contributing guidelines
• [ ] I have tested all aspects of this PR (not required)

Description Allows Lua and C++ to clone RenderTargets. C++ has to delete them, but Lua clones are garbage collected.

I did not test all aspects. I didn't have enough time to test if MIPs worked correctly, but they should if you see how I made the Clone function and given that the base image (MIP 0) works fine.

I used some code that's internal to my codebase but otherwise readable. This code passed.

local display = vgui.Create("RTDisplay")
local rt = vistrace.CreateRenderTarget(512, 512, VisTraceRTFormat.RGBFFF)
rt:Load("512x512_gm_construct_1spp_5bounces_20220924180109.png")
rt:GenerateMIPs()

display:SetSize(512, 512)
display:SetVRT(rt)
display:Update()

timer.Simple(3, function()
    local newRT = rt:Clone()
    display:SetVRT(newRT)
    display:Update()
    print("Cloned!")

    timer.Simple(3, function()
        newRT:Load("512x512_gm_construct_1spp_5bounces_20220923192755.png")
        display:Update()
        timer.Simple(3, function()
            display:Remove()
        end)
    end)
end)

[Derpius]

Quicker to just do it myself in 389c88640c365a8f9df885ed0ded2947dc198a24