Removed joy_masks array.

So far the joy_masks array allowed several joystick drivers for a single target to each have different joy_read return values. However this meant that every call to joy_read implied an additional joy_masks lookup to post-process the return value.

Given that almost all targets only come with a single joystick driver this seems an inappropriate overhead. Therefore now the target header files contain constants matching the return value of the joy_read of the joystick driver(s) on that target.

If there indeed are several joystick drivers for a single target they must agree on a common return value for joy_read. In some cases this was alredy the case as there's a "natural" return value for joy_read. However a few joystick drivers need to be adjusted. This may cause some overhead inside the driver. But that is for sure smaller than the overhead introduced by the joy_masks lookup before.

!!! ToDo !!!

The following three joystick drivers become broken with this commit and need to be adjusted:
- atrmj8.s
- c64-numpad.s
- vic20-stdjoy.s

include/c64.h

@@ -99,6 +99,13 @@
 #define TGI_COLOR_LIGHTBLUE     COLOR_LIGHTBLUE
 #define TGI_COLOR_GRAY3         COLOR_GRAY3
 
+/* Masks for joy_read */
+#define JOY_UP_MASK             0x01
+#define JOY_DOWN_MASK           0x02
+#define JOY_LEFT_MASK           0x04
+#define JOY_RIGHT_MASK          0x08
+#define JOY_BTN_1_MASK          0x10
+
 /* Define hardware */
 #include <_vic2.h>
 #define VIC     (*(struct __vic2*)0xD000)