SVMAllocator is hardcoded using default CommandQueue

When working with the C++ binding header, the class SVMAllocator comes with a constructor accepting a cl::Context parameter.

But when cl::vector with SVMAllocator constructing with default value, it will call a enqueueMapSVM and use cl::CommandQueue::getDefault to do the mapping. It may not associate with the cl::Context which the SVMAllocator belongs.

For example, take a look at this piece of code

```
template
using RWAlloc = cl::SVMAllocator>>;

void DoTest(cl::Platform plat) {
cl::vector devices;
ctx.plat.getDevices(CL_DEVICE_TYPE_ALL, &devices);
cl::Context clctx(devices);
cl::CommandQueue queue{ clctx };

RWAlloc rwalloc(clctx);
std::vector> input(numElements, 1, rwalloc);
...
```

The code crashes when constructing input.
It would be better if SVMAllocator comes with a CommandQueue parameter.

Start by tracing SVMAllocator construction and its enqueueMapSVM path, especially where CommandQueue::getDefault is used during cl::vector allocation. Reproduce the provided example and verify that mapping uses the allocator's associated context or queue without crashing.

Written by the indexing model from the issue text.