Merge pull request opencv#14229 from alalek:ffmpeg_downloader_elevated_run

Author: alalek

3rdparty/ffmpeg/ffmpeg-download.ps1.in

@@ -12,6 +12,22 @@ Write-Output ("=" * 120)
 Write-Output ""
 
 if(![System.IO.File]::Exists($output)) {
+    try {
+        [io.file]::OpenWrite($output).close()
+    } catch {
+        Write-Warning "Unable to write: $output"
+        if (!([Security.Principal.WindowsPrincipal][Security.Principal.WindowsIdentity]::GetCurrent()).IsInRole([Security.Principal.WindowsBuiltInRole] "Administrator")) {
+            Write-Warning "Launching with 'Administrator' elevated privileges..."
+            Pause
+            Start-Process powershell.exe "-NoProfile -ExecutionPolicy Bypass -File `"$PSCommandPath`"" -Verb RunAs
+            exit
+        } else {
+            Write-Output "FATAL: Unable to write with elevated privileges: $output"
+            Pause
+            exit 1
+        }
+    }
+
     try {
         Write-Output ("Downloading: " + $output)
         Import-Module BitsTransfer