Vulkan: Use VK_EXT_provoking_vertex, when available, to get OpenGL/PSP behavior for flat shading #19331
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…sary for the next step.
…s to skip software transform for this case.
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"Provoking vertex" decides which vertex in a triangle gets to decide the color when doing flat shading.
We have a software workaround to match the PSP's (and OpenGL's) behavior, but it's buggy, resulting in #19323 .
Fortunately, there's a pretty widely available Vulkan extension that switches it over to OpenGL's behavior (last-vertex) so if available, let's just use it and skip the software workaround.
This does not fix the bug causing #19323 , but lets us avoid that code. Fixing the bug will be done in a separate PR, which will thus also fix it for D3D9 and D3D11.
The original issue affected by provoking vertex is #10969 .
EDIT: Weirdly, there's still a glitch in Blade Dancer with this.. This glitch is also present in OpenGL.