render/allocator: add udmabuf allocator

udmabuf can create a DMA-BUF backed by a memfd. This is useful
when running with a software implementation of GL/Vulkan: the memfd
can be passed to the parent compositor via wl_shm and the DMA-BUF
can be imported via the usual APIs into GL/Vulkan.

include/render/allocator/udmabuf.h

@@ -0,0 +1,23 @@
+#ifndef RENDER_ALLOCATOR_UDMABUF_H
+#define RENDER_ALLOCATOR_UDMABUF_H
+
+#include <wlr/types/wlr_buffer.h>
+#include <wlr/render/allocator.h>
+
+struct wlr_udmabuf_buffer {
+	struct wlr_buffer base;
+
+	size_t size;
+	struct wlr_shm_attributes shm;
+	struct wlr_dmabuf_attributes dmabuf;
+};
+
+struct wlr_udmabuf_allocator {
+	struct wlr_allocator base;
+
+	int fd;
+};
+
+struct wlr_allocator *wlr_udmabuf_allocator_create(void);
+
+#endif

include/wlr/config.h.in

@@ -9,6 +9,7 @@
 #mesondefine WLR_HAS_VULKAN_RENDERER
 
 #mesondefine WLR_HAS_GBM_ALLOCATOR
+#mesondefine WLR_HAS_UDMABUF_ALLOCATOR
 
 #mesondefine WLR_HAS_XWAYLAND