Finally removed old defunct OpenGL demo
Converted progress bar to using the skin pixmap

  - Port of the progress bar draw function
  - Added stretch blend/blit functions
     - Copy source region to destination region of different size
     - Can either perform image stretching or repeating
     - Usefull for further ports
Moved progress bar & existing button render code to widgets

  - Function "sgui_skin_get_element" returns region on skin pixmap with
    the element in question
  - Widgets compute their size using this function
  - Widgets render themselves using the element regions and blending
Made button and scrollbar use skin pixmap and moved rendering code

  - button and scrollbar use skin pixmap
  - moved rendering code to draw callbacks
  - widget code got messier!