Correct test for GL_EXT_texture_env_add support

The test string is missing the initial "GL_" so it is always failing the test even when supported on newer hardware.

code/sdl/sdl_glimp.c

@@ -627,7 +627,7 @@ static void GLimp_InitExtensions( void )
 
 	// GL_EXT_texture_env_add
 	glConfig.textureEnvAddAvailable = qfalse;
-	if ( SDL_GL_ExtensionSupported( "EXT_texture_env_add" ) )
+	if ( SDL_GL_ExtensionSupported( "GL_EXT_texture_env_add" ) )
 	{
 		if ( r_ext_texture_env_add->integer )
 		{