graphics: draw single indexed indirect draws from GPU-read arguments

src/graphics/host_gpu/renderer/render.h

@@ -57,6 +57,8 @@ struct DrawIndexArgs {
 	uint32_t         first_instance             = 0;
 	DrawOffsetSource offset_source              = DrawOffsetSource::DrawState;
 	uint32_t         render_target_slice_offset = 0;
+	// Nonzero: the GPU reads this VkDrawIndexedIndirectCommand; index_count is INDEX_BUFFER_SIZE.
+	uint64_t indirect_args = 0;
 };
 
 struct DrawAutoArgs {
@@ -163,7 +165,8 @@ public:
 	                    std::span<PreparedBindings* const> bindings);
 
 private:
-	void DrawIndex(uint64_t submit_id, CommandBuffer& buffer, const DrawIndexArgs& args);
+	// False, with nothing drawn, when the shaders cannot take indirect_args from the GPU.
+	bool DrawIndex(uint64_t submit_id, CommandBuffer& buffer, const DrawIndexArgs& args);
 	void DrawAuto(uint64_t submit_id, CommandBuffer& buffer, const DrawAutoArgs& args);
 
 	struct GraphicsBindings {