[al] ColorSet import/export: support USD displayOpacity primvar (#1015)

[robthebloke]

UsdImaging uses displayOpacity in tandem with displayColor to set trasparency on objects.

In AL_USDMaya, the colorset export supported either:

• generic export of rgb/rgba data "float4[] primvars:colorSet1 = [(-1, -1, -1, -1)"
• special case export to the "displayColor" attribute used by Hydra if the the name of the maya colorSet is "displayColor".

This PR adds export of the "displayOpacity" attribute to the second case above, if we're exporting RGBA data

Output (manually simpliifed) now looks like this

    def Mesh "face"
    {   
        color3f[] primvars:displayColor = [(0, 0.8275346, 0.978), (0, 0.19235145, 0.2273255), (0, 0.8275346, 0.978), (0, 0.37345737, 0.44136077), (0, 0.17660311, 0.20871373), (0, 0.15823004, 0.187)] (
            interpolation = "uniform"
        )
        float[] primvars:displayOpacity = [1, 1, 1, 1, 1, 1] (
            interpolation = "uniform"
        )
    } 

[HamedSabri-adsk]

const std::vector<UsdGeomPrimvar> primvars = mesh.GetPrimvars();
for (auto it = primvars.begin(), end = primvars.end(); it != end; ++it)
{
    const UsdGeomPrimvar& primvar = *it;
}

Above code can be refactored out to something a bit nicer:

for (const UsdGeomPrimvar& primvar : mesh.GetPrimvars())
{
}
or even better with auto
for (const auto& primvar : mesh.GetPrimvars())
{
}

[murphyeoin]

Really this is just sugar IMO....in this case I was refactoring existing code and therefore adhering to the existing style.
If it's covered by our upcoming style guide, perfectly happy to adhere to it, but for now I think we should avoid too much stylistic quibbling.(There is debate about whether the use of auto makes things more readable or not - not that I want to enter into it)

[HamedSabri-adsk]

const std::vector<UsdGeomPrimvar> primvars = mesh.GetPrimvars();
for (auto it = primvars.begin(), end = primvars.end(); it != end; ++it)
{
    const UsdGeomPrimvar& primvar = *it;
}

Above code can be refactored out to something a bit nicer:

for (const UsdGeomPrimvar& primvar : mesh.GetPrimvars())
{
}
or even better with auto
for (const auto& primvar : mesh.GetPrimvars())
{
}

[HamedSabri-adsk]

Great, thank you @murphyeoin for the changes. LGTM.

Just a side note, there is a comment in line 276 that says Maya fails when the surface normal
is computed using the left handed rule. I don't have time to look into this further right now but would be nice to have an issue ticket on github to keep track of it.

[murphyeoin]

Thanks @HamedSabri-adsk - we will take a look at the surface normal problem. We've made some small additions/improvements to this PR internally which we were just about to push - will create a separate PR for those