avutil/hwcontext_videotoolbox: Allocate pub and priv frames hwctx together

This is possible because the lifetime of both coincide.
Besides reducing the number of allocations this also simplifies
access to VTFramesContext as one no longer has to
go through AVHWFramesInternal.

Tested-by: Jan Ekström <jeebjp@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavutil/hwcontext_videotoolbox.c

@@ -34,6 +34,10 @@
 #include "pixdesc.h"
 
 typedef struct VTFramesContext {
+    /**
+     * The public AVVTFramesContext. See hwcontext_videotoolbox.h for it.
+     */
+    AVVTFramesContext p;
     CVPixelBufferPoolRef pool;
 } VTFramesContext;
 
@@ -176,12 +180,12 @@ uint32_t av_map_videotoolbox_format_from_pixfmt2(enum AVPixelFormat pix_fmt, boo
 
 static int vt_pool_alloc(AVHWFramesContext *ctx)
 {
-    VTFramesContext *fctx = ctx->internal->priv;
+    VTFramesContext *fctx = ctx->hwctx;
+    AVVTFramesContext *hw_ctx = &fctx->p;
     CVReturn err;
     CFNumberRef w, h, pixfmt;
     uint32_t cv_pixfmt;
     CFMutableDictionaryRef attributes, iosurface_properties;
-    AVVTFramesContext *hw_ctx = ctx->hwctx;
 
     attributes = CFDictionaryCreateMutable(
         NULL,
@@ -237,7 +241,7 @@ static AVBufferRef *vt_pool_alloc_buffer(void *opaque, size_t size)
     AVBufferRef *buf;
     CVReturn err;
     AVHWFramesContext *ctx = opaque;
-    VTFramesContext *fctx = ctx->internal->priv;
+    VTFramesContext *fctx = ctx->hwctx;
 
     err = CVPixelBufferPoolCreatePixelBuffer(
         NULL,
@@ -260,7 +264,7 @@ static AVBufferRef *vt_pool_alloc_buffer(void *opaque, size_t size)
 
 static void vt_frames_uninit(AVHWFramesContext *ctx)
 {
-    VTFramesContext *fctx = ctx->internal->priv;
+    VTFramesContext *fctx = ctx->hwctx;
     if (fctx->pool) {
         CVPixelBufferPoolRelease(fctx->pool);
         fctx->pool = NULL;
@@ -763,10 +767,9 @@ const HWContextType ff_hwcontext_type_videotoolbox = {
     .type                 = AV_HWDEVICE_TYPE_VIDEOTOOLBOX,
     .name                 = "videotoolbox",
 
-    .frames_priv_size     = sizeof(VTFramesContext),
+    .frames_hwctx_size    = sizeof(VTFramesContext),
 
     .device_create        = vt_device_create,
-    .frames_hwctx_size    = sizeof(AVVTFramesContext),
     .frames_init          = vt_frames_init,
     .frames_get_buffer    = vt_get_buffer,
     .frames_get_constraints = vt_frames_get_constraints,