d3dx8: Implement D3DXMatrixStack_MultMatrix.

dlls/d3dx8/math.c

diff --git a/dlls/d3dx8/math.c b/dlls/d3dx8/math.c
index f41c9f4..34c177a 100644
--- a/dlls/d3dx8/math.c
+++ b/dlls/d3dx8/math.c
@@ -686,8 +686,13 @@
 static HRESULT WINAPI ID3DXMatrixStackImpl_MultMatrix(ID3DXMatrixStack *iface, CONST D3DXMATRIX *pm)
 {
     ID3DXMatrixStackImpl *This = (ID3DXMatrixStackImpl *)iface;
-    FIXME("(%p) : stub\n",This);
-    return E_NOTIMPL;
+
+    TRACE("iface %p\n", iface);
+
+    if (!pm) return D3DERR_INVALIDCALL;
+    D3DXMatrixMultiply(&This->stack[This->current], &This->stack[This->current], pm);
+
+    return D3D_OK;
 }
 
 static HRESULT WINAPI ID3DXMatrixStackImpl_MultMatrixLocal(ID3DXMatrixStack *iface, CONST D3DXMATRIX *pm)