app_server: Some cleanup of the new clipping code.

 * Fixed some coding style issues and use regular pointers like everywhere
   else in app_server code.
 * Moved _RenderPicture() from View to AlphaMask.
app_server: Support alpha masks for text rendering...

... both vector and bitmap based. Sub-pixel text rendering not yet handled,
I think the scanline is used differently in this case, in that three times the
horizontal resolution is used, while the alpha map doesn't match this
increase.
app_server: Move AlphaMask management into DrawState.

 * Give DrawState a real copy constructor, handle deriving in PushState().
   (Although clipping region and alpha mask are not cloned, which is on the
   other hand just what's needed for now.)
 * Combining alpha masks from previous states is not yet handled.
 * Remove SetAlphaMask() from DrawingEngine and Painter. It is now done in
   SetDrawState().