Added new option:

In renderwindow, press AKEY to see the alpha in B&W. Another AKEY shows
normal RGB again. This uses glColorMask, an opengl 1.0 function.
Really needed it for debugging alpha in rendering. :)

source/blender/src/space.c

@@ -137,6 +137,8 @@
 
 #include "BLO_readfile.h" /* for BLO_blendhandle_close */
 
+#include "PIL_time.h"
+
 #include "BPY_extern.h"
 
 #include "mydevice.h"