RAT_LBHD() Determines if the mouse left button is currently
 depressed

 Returns
 <lDepressed> => mouse left button is currently
 depressed

 Syntax
 RAT_LBHD()

 Description
 Determines if the mouse left button is currently
 depressed

 Examples
  if RAT_LBHD()
    ?"Mouse left button depressed NOW"
  endif

 Notes:
 This is different from the event function
 RAT_ELBHD(). RAT_ELBHD(<n>) watches the mouse for <n> seconds,
 and if the mouse remains depressed for the full time, then it is
 considered to be HELD DOWN. RAT_LBHD() on the other hand, only
 checks for the mouse button being depressed RIGHT NOW.
 RAT_ELBHD() calls RAT_LBHD() repetitively.

 Source: S_MOUSE.ASM