Added pixmaps

Instead of transfering data all the time when blending or blitting, the
user creates a pixmap that manages data storage on the server side.
The picture is uploaded once and the server side data used for blending
and blitting.

TODO:

  - Xrender expects premultiplied alpha! Looks strange when uploading an
    RGBA image and only performing blitting.
  - Port to WinDOS backend
  - Port to OpenGL backend
Added foolish port of pixmaps to winapi backend

  - Just uses memory buffer with premultiplied alpha values
  - Does not do any clipping, crashes if pixmap goes over window border

  - NEEDS PORT TO ACTUAL GDI BITMAPS
Ported WinDOS pixmap to GDI bitmaps
Moved common OpenGL pixmap code to OpenGL code