Read a keyboard character if one is ready.
sBuffer = Canvas_Inkey
Returns a string of 0, 1, or 2 characters that reflects the status of the keyboard buffer for the selected graphic target
A null string (LEN=0) means that the buffer is empty - no key pressed.
A string length of one means that an ASCII key was pressed and the string contains the ASCII character. An ASCII value between 1 and 31 indicate a control code.
A string length of two means that an extended key was pressed. In this case, the first character in the string has an ASCII value of zero, and the second is the extended keyboard code.