- fixed code dependant on the order of evaluation. code like
  *ptr++ = *ptr++ = *ptr++ = color;
  is wrong, because there is no guarantee that ptr will be incremented
  _between_ the assignment operations. it can be incremented after all 
  assignment operations as well. Because of this both of these are valid
  implementations for a compiler:
   a. assign, increment, assign, increment, assign, increment
   b. assign, assign, assign, increment by 3
  In case b. only the first memory location of the 3 will be modified, being
  assigned 3 times the same value, while the other 2 remain unchanged.
  For example egcs-2.91.66 (and possibly gcc-2.95.x too) implement this in
  the second way (like in case b.)
  Also the order in which the assignement is made is undefined (left to right 
  or right to left).

- added RMakeCenteredImage()

- Added code to draw pixels and lines. Both writing absolute values, or
  offseting existing pixels with delta values are supported.
- Remade the beveling code to be simpler, using the RAlterLine() functions.
- Fixed some pixels in the bevels.
- Changed RAlter* functions to more generic ROperate*
