The `closest` filtertype on images isn't respected in viewers

[expenses]

Hi, I'm running into an issue where image nodes that are set to closest/nearest-neighbour texture filtering still use bilinear filtering in MaterialXView, MaterialXGraphEditor and usdview when USD is compiled with MaterialX support.

I'm not sure if this is the same issue for both MaterialX and USD that can be solved by a fix to MaterialX or if they're seperate.

[jstone-lucasfilm]

Good catch, @expenses, and this looks like an oversight that needs to be addressed in MaterialXRender and MaterialXRenderGlsl.

[mapetikea]

I think that the bug is also present in the OSL codegen. It doesn't seem to respect filters either?