Merge a UsdTexture/UsdTransform2d/UsdPrimvarReader_float2 chain into a single image node

**Describe the feature you'd like**
The Arnold image node is able access UV coordinates and apply basic texture transformations, so we should be able to collapse a chain of texture<>transform<>primvar reader nodes onto a single image node, like what we do with a texture <> primvar reader nodes for now. Without merging these nodes, the image sampler does not make use of texture derivatives, so texture sampling is going to be less efficient.

Start by tracing the existing texture-to-primvar reader merge mentioned in the issue, then compare it with the UsdTexture, UsdTransform2d, and UsdPrimvarReader_float2 chain. Done means the chain is represented by one Arnold image node that applies the texture coordinates and transformations while preserving texture-derivative-based sampling.

Written by the indexing model from the issue text.