input: fix up usage of button->down, used to be a bitmask, is now an array.

device->button->down used to be a 32-byte bitmask with one bit for each
button. This has changed into a 256-byte array, with one byte assigned for
each button. Some of the callers were still using this array as a bitmask
however, this is fixed with this patch.

Thanks to Keith Packard for pointing this out. See also:
http://lists.freedesktop.org/archives/xorg/2008-June/036202.html

include/inputstr.h

@@ -57,6 +57,9 @@ SOFTWARE.
 #include "privates.h"
 
 #define BitIsOn(ptr, bit) (((BYTE *) (ptr))[(bit)>>3] & (1 << ((bit) & 7)))
+/* If byte[i] in src is non-zero, set bit i in dst, otherwise set bit to 0 */
+#define SetBitIf(dst, src, i) \
+    (src[i]) ? (dst[i/8] |= (1 << (i % 8))) : (dst[i/8] &= ~(1 << (i % 8)));
 
 #define SameClient(obj,client) \
 	(CLIENT_BITS((obj)->resource) == (client)->clientAsMask)