Rendering directly to a resource using OpenGL interop and Hgi
doesn't work in Houdini, since it never uses the resulting resource
(it does not call `HdRenderBuffer::GetResource`). But since doing
that simultaneously disables mapping (`HdRenderBuffer::Map` is
not implemented then), nothing was displayed. To fix this, keep
track of whether a Hydra viewport does support displaying a Hgi
resource directly, by checking whether
`HdRenderBuffer::GetResource` is ever called and only enable use
of OpenGL interop if that is the case.
Differential Revision: https://developer.blender.org/D15090
34f94a02f3