lavu: add Vulkan hwcontext code

This commit adds the necessary code to initialize and use a Vulkan device
within the hwcontext libavutil framework.
Currently direct mapping to VAAPI and DRM frames is functional, and
transfers to CUDA and native frames are supported.

Lets hope the future Vulkan video decode extension fits well within this
framework.

libavutil/hwcontext.c

@@ -58,6 +58,9 @@ static const HWContextType * const hw_table[] = {
 #endif
 #if CONFIG_MEDIACODEC
     &ff_hwcontext_type_mediacodec,
+#endif
+#if CONFIG_VULKAN
+    &ff_hwcontext_type_vulkan,
 #endif
     NULL,
 };
@@ -73,6 +76,7 @@ static const char *const hw_type_names[] = {
     [AV_HWDEVICE_TYPE_VDPAU]  = "vdpau",
     [AV_HWDEVICE_TYPE_VIDEOTOOLBOX] = "videotoolbox",
     [AV_HWDEVICE_TYPE_MEDIACODEC] = "mediacodec",
+    [AV_HWDEVICE_TYPE_VULKAN] = "vulkan",
 };
 
 enum AVHWDeviceType av_hwdevice_find_type_by_name(const char *name)