Add XAudio2 redistributable DLL for older Windows versions

The CI builds link against XAudio2_9, which isn't available on Windows 8
and lower. As a quick and dirty workaround, this commit ships the
XAudio2 redistributable on those older Windows versions. Simply renaming
the redistributable to XAudio2_9.dll isn't strictly allowed by the
documentation, but it works and is probably an acceptable hack to
support an outdated operating system...

Fixes arch1t3cht/Aegisub#16 .

packages/win_installer/fragment_runtimes.iss

@@ -2,6 +2,7 @@
 
 [Files]
 DestDir: {tmp}; Source: "{#DEPS_DIR}\VC_redist\VC_redist.x{#ARCH}.exe"; Flags: nocompression deleteafterinstall
+DestDir: {app}; Source: "{#DEPS_DIR}\XAudio2_redist\build\native\release\bin\x{#ARCH}\xaudio2_9redist.dll"; DestName: "XAudio2_9.dll"; OnlyBelowVersion: 10.0
 
 [Run]
 Filename: {tmp}\VC_redist.x{#ARCH}.exe; StatusMsg: {cm:InstallRuntime}; Parameters: "/install /quiet /norestart"

packages/win_installer/portable/create-portable.ps1

@@ -60,6 +60,9 @@ Write-Output 'Copying - codecs\VSFilter'
 Copy-New-Item $InstallerDepsDir\VSFilter\x64\VSFilter.dll  $PortableOutputDir\csri
 Write-Output 'Copying - runtimes\MS-CRT'
 Copy-New-Item $InstallerDepsDir\VC_redist\VC_redist.x64.exe $PortableOutputDir\Microsoft.CRT
+Write-Output 'Copying - redist\XAudio2_9'
+Copy-New-Item $InstallerDepsDir\XAudio2_redist\build\native\release\bin\x64\xaudio2_9redist.dll $PortableOutputDir\Redist
+Rename-Item $PortableOutputDir\Redist\xaudio2_9redist.dll $PortableOutputDir\Redist\XAudio2_9.dll
 
 Write-Output 'Copying - automation'
 Copy-New-Items "$InstallerDir\share\aegisub\automation\*"  "$PortableOutputDir\automation\"  -Recurse

tools/win-installer-setup.ps1

@@ -74,6 +74,14 @@ if (!(Test-Path VC_redist)) {
 	Invoke-WebRequest https://aka.ms/vs/16/release/VC_redist.x64.exe -OutFile "$redistDir\VC_redist.x64.exe" -UseBasicParsing
 }
 
+# XAudio2 redistributable
+if (!(Test-Path XAudio2_redist)) {
+	New-Item -ItemType Directory XAudio2_redist
+	Invoke-WebRequest https://www.nuget.org/api/v2/package/Microsoft.XAudio2.Redist/1.2.11 -OutFile XAudio2Redist.zip
+	Expand-Archive -LiteralPath XAudio2Redist.zip -DestinationPath XAudio2_redist
+	Remove-Item XAudio2Redist.zip
+}
+
 # TODO dictionaries
 
 # localization