Fixed crash if initialization of EGL failed but was tried again later.

The internal function SDL_EGL_LoadLibrary() did not delete and remove a mostly
uninitialized data structure if loading the library first failed. A later try to
use EGL then skipped initialization and assumed it was previously successful
because the data structure now already existed. This led to at least one crash
in the internal function SDL_EGL_ChooseConfig() because a NULL pointer was
dereferenced to make a call to eglBindAPI().

build-scripts/winrtbuild.bat

@@ -0,0 +1,8 @@
+@ECHO OFF
+REM
+REM winrtbuild.bat: a batch file to help launch the winrtbuild.ps1
+REM   Powershell script, either from Windows Explorer, or through Buildbot.
+REM
+SET ThisScriptsDirectory=%~dp0
+SET PowerShellScriptPath=%ThisScriptsDirectory%winrtbuild.ps1
+PowerShell -NoProfile -ExecutionPolicy Bypass -Command "& '%PowerShellScriptPath%'";