DPaint IV enhanced color cycle chunk (EA)

DRNG Chunk for FORM ILBM
========================

Submitted by Lee Taran

Purpose:

Enhanced Color Cycling Capabilities
-------------------------------------
   * DPaintIV supports a new color cycling model which does NOT
     require that color cycles contain a contiguous range of color
     registers.

     For example:
       If your range looks like:  [1][3][8][2]
       then at each cycle tick
          temp = [2], 
          [2] = [8],
          [8] = [3],
          [3] = [1], 
          [1] = temp

   * You can now cycle a single register thru a series of rgb values.
     For example:
        If your range looks like: [1] [orange] [blue] [purple]
        then at each cycle tick color register 1 will take on the
        next color in the cycle.
         
        ie:  t=0:  [1] = curpal[1]
             t=1:  [1] = purple
             t=2:  [1] = blue
             t=3:  [1] = orange
             t=4:  goto t=0

   * You can combine rgb cycling with traditional color cycling.
     For example:
         Your range can look like:
             [1] [orange] [blue] [2] [green] [yellow]

         t=0: [1] = curpal[1], [2] = curpal[2]
         t=1: [1] = yellow,    [2] = blue
         t=2: [1] = green,     [2] = orange
         t=3: [1] = curpal[2], [2] = curpal[1]
         t=4: [1] = blue,      [2] = yellow
         t=5: [1] = orange,    [2] = green
         t=6: goto t=0

Note:
   * DPaint will save out an old style range CRNG if the range fits
     the CRNG model otherwise it will save out a DRNG chunk. 
   * no thought has been given (yet) to interlocking cycles



/* --------------------------------------------------------------------- 

 IFF Information:  DPaintIV DRNG chunk

          DRNG ::= "DRNG" # { DRange DColor* DIndex* }
                                                                      
 a <cell> is where the color or register appears within the range     

 The RNG_ACTIVE flags is set when the range is cyclable. A range 
 should only have the RNG_ACTIVE if it:
      1> contains at least one color register
      2> has a defined rate 
      3> has more than one color and/or color register
 If the above conditions are met then RNG_ACTIVE is a user/program 
 preference.  If the bit is NOT set the program should NOT cycle the
 range.

 The RNG_DP_RESERVED flags should always be 0!!!
 --------------------------------------------------------------------- */
typedef struct {
   UBYTE min;           /* min cell value */
   UBYTE max;           /* max cell value */
   SHORT rate;          /* color cycling rate, 16384 = 60 steps/second */
   SHORT flags;         /* 1=RNG_ACTIVE,4=RNG_DP_RESERVED */
   UBYTE ntrue;         /* number of DColor structs to follow */
   UBYTE nregs;         /* number of DIndex structs to follow */
   } DRange;           

typedef struct { UBYTE cell; UBYTE r,g,b; } DColor; /* true color cell */
typedef struct { UBYTE cell; UBYTE index; } DIndex; /* color register cell */