D3D: Fix EFB depth buffer copies, filtering on scaled EFB copies when MSAA is enabled, real XFB filtering

Since ResolveSubresource cannot be used with depth textures (and throws an error with the debug layer enabled), use a shader which selects the minimum depth value from all samples.

Changes the sampler by XFBEncoder to use a linear filter, rather than point, to match GL behavior.

Source/Core/VideoBackends/D3D/XFBEncoder.cpp

@@ -247,8 +247,7 @@ void XFBEncoder::Init()
 	// Create EFB texture sampler
 
 	D3D11_SAMPLER_DESC sd = CD3D11_SAMPLER_DESC(CD3D11_DEFAULT());
-	// FIXME: Should we really use point sampling here?
-	sd.Filter = D3D11_FILTER_MIN_MAG_MIP_POINT;
+	sd.Filter = D3D11_FILTER_MIN_MAG_LINEAR_MIP_POINT;
 	hr = D3D::device->CreateSamplerState(&sd, &m_efbSampler);
 	CHECK(SUCCEEDED(hr), "create xfb encode texture sampler");
 	D3D::SetDebugObjectName(m_efbSampler, "xfb encoder texture sampler");