Made get_key_if_available actually work.

On the A2 get_key deletes any already pressed key. Therefore it's a bad idea(tm) to call it to retrieve the key detected in get_key_if_available. So now get_key_if_available insteads retrieves the key on its own via direct hw access. This was already done the very same way in check_for_abort_key. And apart from not working as expected the benefit of get_key displaying a cursor wasn't useful here anyway because get_key was supposed to return right away with the pressed key so the cursor wouldn't have been visible at all.

drivers/a2input.s

@@ -17,10 +17,13 @@ get_key:
 ; inputs: none
 ; outputs: A contains ASCII value of key just pressed (0 if no key pressed)
 get_key_if_available:
-  bit $c000                     ; key down?
-  bmi get_key
+  lda $c000                     ; current key pressed
+  bmi :+
   lda #0
   rts
+: bit $c010                     ; clear the keyboard strobe
+  and #$7f
+  rts
 
 ; check whether the escape key is being pressed
 ; inputs: none