backend: stop using renderer to get the buffer type

When picking a format, the backend needs to know whether the
buffers allocated by the allocator will be DMA-BUFs or shared
memory. So far, the backend used the renderer's supported
buffer types to guess this information.

This is pretty fragile: renderers in general don't care about the
SHM cap (they only care about the DATA_PTR one). Additionally,
nothing stops a renderer from supporting both DMA-BUFs and shared
memory, but this would break the backend's guess.

Instead, use wlr_allocator.buffer_caps. This is more reliable since
the buffers created with the allocator are guaranteed to have these
caps.

backend/wayland/backend.c

@@ -17,6 +17,7 @@
 
 #include "backend/backend.h"
 #include "backend/wayland.h"
+#include "render/allocator.h"
 #include "render/drm_format_set.h"
 #include "render/pixel_format.h"
 #include "render/wlr_renderer.h"
@@ -451,11 +452,10 @@ struct wlr_backend *wlr_wl_backend_create(struct wl_display *display,
 		goto error_drm_fd;
 	}
 
-	uint32_t caps = renderer_get_render_buffer_caps(renderer);
 	const struct wlr_drm_format_set *remote_formats;
-	if ((caps & WLR_BUFFER_CAP_DMABUF) && wl->zwp_linux_dmabuf_v1) {
+	if ((allocator->buffer_caps & WLR_BUFFER_CAP_DMABUF) && wl->zwp_linux_dmabuf_v1) {
 		remote_formats = &wl->linux_dmabuf_v1_formats;
-	} else if ((caps & WLR_BUFFER_CAP_DATA_PTR) && wl->shm) {
+	} else if ((allocator->buffer_caps & WLR_BUFFER_CAP_SHM) && wl->shm) {
 		remote_formats = &wl->shm_formats;
 	}  else {
 		wlr_log(WLR_ERROR,

backend/x11/backend.c

@@ -32,9 +32,8 @@
 
 #include "backend/backend.h"
 #include "backend/x11.h"
+#include "render/allocator.h"
 #include "render/drm_format_set.h"
-#include "render/gbm_allocator.h"
-#include "render/shm_allocator.h"
 #include "render/wlr_renderer.h"
 #include "types/wlr_buffer.h"
 #include "util/signal.h"
@@ -619,11 +618,10 @@ struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
 		goto error_event;
 	}
 
-	uint32_t caps = renderer_get_render_buffer_caps(renderer);
 	const struct wlr_drm_format_set *pixmap_formats;
-	if (x11->have_dri3 && (caps & WLR_BUFFER_CAP_DMABUF)) {
+	if (x11->have_dri3 && (allocator->buffer_caps & WLR_BUFFER_CAP_DMABUF)) {
 		pixmap_formats = &x11->dri3_formats;
-	} else if (x11->have_shm && (caps & WLR_BUFFER_CAP_DATA_PTR)) {
+	} else if (x11->have_shm && (allocator->buffer_caps & WLR_BUFFER_CAP_SHM)) {
 		pixmap_formats = &x11->shm_formats;
 	} else {
 		wlr_log(WLR_ERROR,