Expose UsdImagingGLEngineWrapper's `GetAovRenderBuffer(_:)` to Swift

Currently, it is not possible to call the `GetAovRenderBuffer(_:)` instance method on `UsdImagingGLEngineWrapper` in Swift, which otherwise currently results in the following compiler error:
```pwsh
Value of type 'Overlay.UsdImagingGLEngineWrapper' has no member 'GetAovRenderBuffer'

C++ method 'GetAovRenderBuffer' that returns a pointer of type 'Optional' is unavailable
C++ method 'GetAovRenderBuffer' may return an interior pointer

_OpenUSD_SwiftBindingHelpers/include/swiftUsd/Wrappers/UsdImagingGLEngineWrapper.h:405:30:
Annotate method 'GetAovRenderBuffer' with 'SWIFT_RETURNS_INDEPENDENT_VALUE' in C++ to make it available in Swift
```

I would be happy to make a PR for this, if you'd like to expose this to Swift.

Start in _OpenUSD_SwiftBindingHelpers/include/swiftUsd/Wrappers/UsdImagingGLEngineWrapper.h at line 405, where GetAovRenderBuffer is declared. Follow the compiler diagnostic's guidance for the required annotation, then verify that the method is callable from Swift without the reported unavailable-method error.

Written by the indexing model from the issue text.