- implement r_noborder, inspired by patch from ensiform (#4289)

- Add Sys_SetEnv for portable setting of environment variables
 - Default ioquake3 to appear on the center of the screen if running in windowed mode.

code/sys/sys_win32.c

@@ -652,3 +652,16 @@ void Sys_PlatformInit( void )
 		SDL_VIDEODRIVER_externallySet = qfalse;
 #endif
 }
+
+/*
+==============
+Sys_SetEnv
+
+set/unset environment variables (empty value removes it)
+==============
+*/
+
+void Sys_SetEnv(const char *name, const char *value)
+{
+	_putenv(va("%s=%s", name, value);
+}