[stephanmeesters]

Runs the rendering tests from ci/rendering-tests on bobtista's trunk for four backends and shows their images side by side: DX8, and bgfx with Direct3D 11, Direct3D 12 and Vulkan.

The base branch bobtista-trunk is a copy of bobtista/topic/trunk at e7e111c. trunk has no googletest setup, so this adds only what the rendering tests need instead of squashing the upstream commits it is missing.

• Tests: the fixture now goes through the render backend instead of raw D3D8. It clears with IRenderBackend::Clear, draws the triangle with Render2DClass, and reads the image back with Capture_Back_Buffer_Image on DX8 or bgfx's own screenshot.
• Workflow: check-rendering.yml runs on pull requests. It builds z_googletest three times (x86: DX8, bgfx dx11, bgfx vulkan); Direct3D 12 runs the dx11 build, because bgfx gives both the same shaders. It runs DX8 on DXVK and lavapipe, Vulkan on lavapipe, and Direct3D 11 and 12 on WARP.
• Report: one row per test and one column per backend, with each bgfx image's difference from DX8. The check fails when a backend's tests fail; differences alone don't fail it.

Found while running it locally under Wine (to be confirmed here on Windows):

• bgfx ignores the clear color on the back buffer and clears to black, so the Clear test and the backgrounds of the blending, texture and text tests differ.
• The laser and the particles render nothing on bgfx until an earlier frame drew a textured 2D quad.
• bgfx draws the 2D shapes one pixel off from DX8.

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