Add getCurrentCommandBuffer for all frame workloads

Rather than allocating new command buffers for each small task, schedule
all work onto the frame's main command buffer to be dispatched at
display-time. Fixes all layout transition and synchronization issues.

include/renderer_vk/renderer_vk.hpp

@@ -50,6 +50,8 @@ class RendererVK final : public Renderer {
 	std::vector<std::vector<vk::UniqueFramebuffer>> frameFramebuffers = {};
 	std::vector<vk::UniqueCommandBuffer> frameGraphicsCommandBuffers = {};
 
+	const vk::CommandBuffer& getCurrentCommandBuffer() const { return frameGraphicsCommandBuffers[frameBufferingIndex].get(); }
+
 	// Todo:
 	// Use `{colourBuffer,depthBuffer}Loc` to maintain an std::map-cache of framebuffers
 	struct Texture {