[libpng16] projects: Add a build launcher to the Visual Studio project

Add projects/vstudio/build.bat, a wrapper for `devenv [...] /build`.

This is a cherry-pick of commit f1c01b5
from branch 'libpng18'.

projects/vstudio/build.bat

@@ -0,0 +1,25 @@
+@echo off
+@setlocal enableextensions
+
+if "%~1" == "/?" goto :help
+if "%~1" == "-?" goto :help
+if "%~1" == "/help" goto :help
+if "%~1" == "-help" goto :help
+if "%~1" == "--help" goto :help
+goto :run
+
+:help
+echo Usage:
+echo   %~nx0 [SOLUTION_CONFIG]
+echo Examples:
+echo   %~nx0 "Release|Win32" (default)
+echo   %~nx0 "Debug|Win32"
+echo   %~nx0 "Release|ARM64"
+echo   %~nx0 "Debug|ARM64"
+echo   etc.
+exit /b 2
+
+:run
+set _SOLUTION_CONFIG="%~1"
+if %_SOLUTION_CONFIG% == "" set _SOLUTION_CONFIG="Release|Win32"
+devenv "%~dp0.\vstudio.sln" /build %_SOLUTION_CONFIG%