Merged skin subsystem widget extent function

  - One function returning a rect for all widget types
  - text extent function returns a rect too
  - widget code is for now simply a port
Simplified some widgets, made skinning functions accept rects
Implemented skin pixmap stub

  - canvas has a skin pixmap
  - X11 implementation has a global pixmap
  - Skinning subsystem loads skin to pixmap
  - Skinning subsystem uses pixmap to draw widgets

  - Fixed pixmap blending/blitting
    - Common canvas code had wrong clipping implementation
    - X11 canvas had offset wrongfully added
Added skin pixmap implementation to OpenGL compatibillity canvas
Added WinDOS implementation of skin pixmap