Backends: Vulkan: Add override for the subpass to reference during VkPipeline creation. (#3579)

This allows for binding the pipeline/sending draw commands
(via `ImGui_ImplVulkan_RenderDrawData`) against any subpass, rather than
being restricted to only the first subpass.

Without this, attempting to bind the pipeline against a subpass other than
the first one results in validation layer errors and, at worst, some drivers
failing if the subpass attachments differ.
3 files changed