Fixes Mario Tennis Gimmick Courts and adds support for FastDepthCalc

- Calculate ZSlope every flush but only set PixelShader Constant on Reset Buffer when zfreeze
- Fixed another Pixel Shader bug in D3D that was giving me grief

Source/Core/VideoBackends/D3D/VertexManager.cpp

@@ -181,12 +181,10 @@ void VertexManager::vFlush(bool useDstAlpha)
 
 	PrepareDrawBuffers(stride);
 
-	if (!bpmem.genMode.zfreeze && IndexGenerator::GetIndexLen() >= 3)
-	{
+	if (!bpmem.genMode.zfreeze)
 		CalculateZSlope(stride);
-	}
 
-	// if cull mode is CULL_ALL, ignore triangles and quads
+	// If cull mode is CULL_ALL, do not render these triangles
 	if (bpmem.genMode.cullmode == GenMode::CULL_ALL && current_primitive_type == PRIMITIVE_TRIANGLES)
 		return;
 
@@ -202,6 +200,9 @@ void VertexManager::ResetBuffer(u32 stride)
 {
 	s_pCurBufferPointer = s_pBaseBufferPointer;
 	IndexGenerator::Start(GetIndexBuffer());
+
+	if (bpmem.genMode.zfreeze)
+		PixelShaderManager::SetZSlope(ZSlope.dfdx, ZSlope.dfdy, ZSlope.f0);
 }
 
 }  // namespace