Transforms not working (everything is identity) in testshade --optix

### Problem

Transforms don't appear to work in optix. I'm running the following shader through testshade:
```
surface test_add(output color Cout = 0) {
point Po = transform("myspace", vector(1, 1, 1));
if (u < 0.1 && v < 0.1) {
printf("Po = %f %f %f\n", Po.x, Po.y, Po.z);
}
Ci = diffuse(N) * color(Po) + emission() * u;
}
```
EDIT: I've just realised a my shader is passing a vector to transform() and expecting a point back again, which seems wrong. In any case, if I change that to point(1,1,1) instead of vector the behaviour on both CPU and GPU is unchanged.

**Expected behavior:**
This should print out:
```
Po = 1.0 0.5 1.0
```
and indeed it does when running on the CPU

**Actual behavior:**
Prints:
```
Po = 1.0 1.0 1.0
```
when running with --optix

### Steps to Reproduce

1. Compile the shader above
2. Run `testshade test_add --res 16 16`
3. Run `testshade test_add --res 16 16 --optix`

### Versions

* OSL branch/version: master 17a17191dfc
* OS: ubuntu 18.04
* C++ compiler: gcc-8.4
* LLVM version: 10.0.1
* OIIO version: 2.2.8
* CUDA version: 10.1
* OptiX version: 7.1

Start with the testshade entry point and reproduce the shader using transform("myspace", point(1, 1, 1)) on CPU and with --optix. Trace why the OptiX result remains 1.0 1.0 1.0 instead of the CPU result 1.0 0.5 1.0, then verify the corrected behavior with the reported testshade commands.

Written by the indexing model from the issue text.