configure: make swscale_vulkan depend on unstable

The SPIR-V backend is built on top of the ops code, which is only compiled
with CONFIG_UNSTABLE. Express this dependency in configure instead of
guarding the vulkan Makefile include and ff_sws_vk_init() call separately.

This also fixes linking with --disable-unstable when vulkan is enabled, as
graph.c referenced ff_sws_vk_device_ref() from the excluded vulkan objects.

Fixes: 51d4406e07 ("swscale/graph: support allocating hardware intermediate frames")
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Niklas Haas <git@haasn.dev>

configure

@@ -3354,7 +3354,7 @@ vulkan_deps="threads"
 vulkan_deps_any="libdl LoadLibrary"
 spirv_headers_spirv_h_deps="vulkan"
 spirv_unified1_spirv_h_deps="vulkan"
-swscale_vulkan_deps="vulkan"
+swscale_vulkan_deps="vulkan unstable"
 swscale_vulkan_if_any="spirv_headers_spirv_h spirv_unified1_spirv_h"
 
 apv_vulkan_hwaccel_deps="vulkan spirv_compiler"

libswscale/Makefile

@@ -38,9 +38,7 @@ OBJS-$(CONFIG_UNSTABLE) +=                              \
        uops.o                                           \
        uops_backend.o                                   \
 
-ifeq ($(CONFIG_UNSTABLE),yes)
 include $(SRC_PATH)/libswscale/vulkan/Makefile
-endif
 
 # Objects duplicated from other libraries for shared builds
 SHLIBOBJS                    += framepool.o log2_tab.o half2float.o

libswscale/swscale.c

@@ -1580,7 +1580,7 @@ int sws_frame_setup(SwsContext *ctx, const AVFrame *dst, const AVFrame *src)
         if (dev_ctx->type != AV_HWDEVICE_TYPE_VULKAN)
             return AVERROR(ENOTSUP);
 
-#if CONFIG_UNSTABLE && CONFIG_SWSCALE_VULKAN
+#if CONFIG_SWSCALE_VULKAN
         ret = ff_sws_vk_init(ctx, src_hwfc->device_ref);
         if (ret < 0)
             return ret;