PointInstancers don't respect transform stack

[guidoq]

Description of Issue

The simple USDA shows the problem. The two triangles should be in the same spot (the base should be coincident).

Steps to Reproduce

Render the following. One triangle get the root transformation of (90,0,0) but not the other.

#usda 1.0
(
defaultPrim = "RootXform"
metersPerUnit = 0.01
upAxis = "Y"
)
def Xform "RootXform"
{
def PointInstancer "top_instancer"
{
int64[] ids = [0]
quath[] orientations = [(0, 0, 0, 0)]
point3f[] positions = [(0, 0, 0)]
int[] protoIndices = [0]
prepend rel prototypes = </RootXform/top_instancer/Prototypes/inner_xform>

    def Scope "Prototypes"
    {
        def Xform "inner_xform"
        {
            double3 xformOp:translate = (90, 0, 0)
            uniform token[] xformOpOrder = ["xformOp:translate"]

            def Mesh "blue_triangle"
            {
                int[] faceVertexCounts = [3]
                int[] faceVertexIndices = [0, 1, 2]
                point3f[] points = [(-40, 0, 0), (40, 0, 0), (-40, 80, 0)]
                uniform token subdivisionScheme = "none"
                color3f[] primvars:displayColor = [(0, 0, 1)]( interpolation = "constant" )
            }

            def PointInstancer "nested_instancer"
            {
                int64[] ids = [0]
                quath[] orientations = [(0, 0, 0, 0)]
                point3f[] positions = [(0, 0, 0)]
                int[] protoIndices = [0]
                prepend rel prototypes = </RootXform/top_instancer/Prototypes/inner_xform/nested_instancer/Prototypes/green_triangle>

                def Scope "Prototypes"
                {
                    def Mesh "green_triangle"
                    {
                        int[] faceVertexCounts = [3]
                        int[] faceVertexIndices = [0, 1, 2]
                        point3f[] points = [(-40, 0, 0), (40, 0, 0), (40, 80, 0)]
                        uniform token subdivisionScheme = "none"
                        color3f[] primvars:displayColor = [(0, 1, 0)]( interpolation = "constant" )
                    }
                }
            }
        }
    }
}
def Mesh "red_square"
{
    int[] faceVertexCounts = [4]
    int[] faceVertexIndices = [0, 1, 2, 3]
    point3f[] points = [(-40, 0, 40), (40, 0, 40), (40, 0, -40), (-40, 0, -40)]
color3f[] primvars:displayColor = [(1, 0, 0)]( interpolation = "constant" )
}

}

[tallytalwar]

Filed as internal issue #USD-8145