Backends: Dx12: Enable swapchain tearing to eliminate viewports framerate throttling

[RT2Code]

According to the DXGI documentation, with VSync off and a swapchain using flip swap effects, it is recommended to enable tearing to support displays with variable refresh rate.

Another reason to enable swapchain tearing is that without it, non-fullscreen swapchains are throttled to the display’s refresh rate even with VSync disabled. Enabling tearing removes this limitation and lets viewports run at full framerate.

I used this sample as a reference for this PR.

Please note that fullscreen is not supported yet (no change here, it's already unsupported), that's something I'll fix in a future PR.

[ocornut]

Thanks for your PR.
Some observations:

• We recently started using DXGI_MWA_NO_ALT_ENTER (15c88c4, 3d937be) and I believe your latest merge introduce two spots doing the same thing.
• bd->tearingSupport is recomputed every time in ImGui_ImplDX12_CreateWindow(), could that be moved to Init ?
• In main.cpp, g_tearingSupport -> g_SwapChainTearingSupport.

[ocornut]

Another reason to enable swapchain tearing is that without it, non-fullscreen swapchains are throttled to the display’s refresh rate even with VSync disabled. Enabling tearing removes this limitation and lets viewports run at full framerate.

(For the records, I don't get display refresh rate throttling on my desktop Windows 10 setup.)

[RT2Code]

bd->tearingSupport is recomputed every time in ImGui_ImplDX12_CreateWindow(), could that be moved to Init ?

Correct, and in fact, you made me realize it's also true for the dxgi factory, there's no need to recreate it for each viewport. I applied your suggestions.

(For the records, I don't get display refresh rate throttling on my desktop Windows 10 setup.)

Maybe it's specific to Windows 11? When I disable VSync and no support for tearing, my framerate is capped to 144fps, which is my screen refresh rate.

[ocornut]

Merged as 3dafd9e, b4514ce. Thank you!