hwcontext_vulkan: do not OR the user-specified usage with our default flags

Some users may need special formats that aren't available when the STORAGE
flag bit is set, which would result in allocations failing.

libavutil/hwcontext_vulkan.c

@@ -1703,7 +1703,8 @@ static int vulkan_frames_init(AVHWFramesContext *hwfc)
     hwctx->tiling = hwctx->tiling ? hwctx->tiling : p->use_linear_images ?
                     VK_IMAGE_TILING_LINEAR : VK_IMAGE_TILING_OPTIMAL;
 
-    hwctx->usage |= DEFAULT_USAGE_FLAGS;
+    if (!hwctx->usage)
+        hwctx->usage = DEFAULT_USAGE_FLAGS;
 
     err = create_exec_ctx(hwfc->device_ctx, &fp->conv_ctx,
                           dev_hwctx->queue_family_comp_index,

libavutil/hwcontext_vulkan.h

@@ -112,8 +112,8 @@ typedef struct AVVulkanFramesContext {
      */
     VkImageTiling tiling;
     /**
-     * Defines extra usage of output frames. This is bitwise OR'd with the
-     * standard usage flags (SAMPLED, STORAGE, TRANSFER_SRC and TRANSFER_DST).
+     * Defines extra usage of output frames. If left as 0, the following bits
+     * are set: TRANSFER_SRC, TRANSFER_DST. SAMPLED and STORAGE.
      */
     VkImageUsageFlagBits usage;
     /**