Windows DX12 imgui: provide SrvDescriptorFreeFn and SrvDescriptorAllocFn for "upcoming features"

Imgui 1.91.6 wants the user to provide SRV descriptor alloc/free functions saying that "upcoming features will want multiple" (https://github.com/ocornut/imgui/releases/tag/v1.91.6). Right now, we are only providing one SRV descriptor via the legacy API. It is unclear what the impact of continuing to use the legacy API is (likely, dynamic fonts from 1.92.0 wouldn't work as expected)