Provoking vertex (software transform): Simpler solution

Simply rotate each primitive in the index buffer to simulate a different provoking vertex.

Since at this point we have already generated a plain primitive index
buffer, it's easy to manipulate like this.

An even better solution would be to generate rotated index buffers
directly during decode, although that code is super critical and does
not need more complexity..

We could now also enable this for hardware transform but I'm leaving
that for later.

GPU/Vulkan/DrawEngineVulkan.cpp

@@ -405,11 +405,13 @@ void DrawEngineVulkan::DoFlush() {
 		// do not respect scissor rects.
 		params.allowClear = framebufferManager_->UseBufferedRendering();
 		params.allowSeparateAlphaClear = false;
-		if (renderManager->GetVulkanContext()->GetDeviceFeatures().enabled.provokingVertex.provokingVertexLast) {
-			// We can get the OpenGL behavior, no need for workarounds.
-			params.provokingVertexLast = true;
-		} else {
-			params.provokingVertexLast = false;
+
+		if (gstate.getShadeMode() == GE_SHADE_FLAT) {
+			if (!renderManager->GetVulkanContext()->GetDeviceFeatures().enabled.provokingVertex.provokingVertexLast) {
+				// If we can't have the hardware do it, we need to rotate the index buffer to simulate a different provoking vertex.
+				// We do this before line expansion etc.
+				IndexBufferProvokingLastToFirst(prim, inds, vertexCount);
+			}
 		}
 		params.flippedY = true;
 		params.usesHalfZ = true;