most unused arg warnings corrected.

- removed deprecated bitmap arg from IMB_allocImBuf (plugins will need updating).
- mostly tagged UNUSED() since some of these functions look like they may need to have the arguments used later.

source/blender/blenpluginapi/intern/pluginapi.c

@@ -101,10 +101,9 @@ LIBEXPORT void freeT(void *vmemh)
 LIBEXPORT struct ImBuf *allocImBuf(short x,
 						 short y,
 						 uchar d,
-						 uint flags,
-						 uchar bitmap)
+						 uint flags)
 {
-	return IMB_allocImBuf(x, y, d, flags, bitmap);
+	return IMB_allocImBuf(x, y, d, flags);
 }