hwdec_vulkan: enable AV1 support if available

AV1 support in Vulkan is extremely bleeding edge - to the point that
the extension is not present in official Khronos releases, but it has
a reserved identifier and we can look it up with a string literal for
now.

This will be skipped and ignored if the driver doesn't support it, so
it's safe if/when the name changes later (it'll just never be activated
in that case).

video/out/vulkan/context.c

@@ -177,6 +177,8 @@ bool ra_vk_ctx_init(struct ra_ctx *ctx, struct mpvk_ctx *vk,
         VK_KHR_VIDEO_DECODE_H264_EXTENSION_NAME,
         VK_KHR_VIDEO_DECODE_H265_EXTENSION_NAME,
         VK_KHR_VIDEO_QUEUE_EXTENSION_NAME,
+        // This is a literal string as it's not in the official headers yet.
+        "VK_MESA_video_decode_av1",
     };
 
     VkPhysicalDeviceDescriptorBufferFeaturesEXT descriptor_buffer_feature = {