Introduce Emacs_Pix_Container and Emacs_Pix_Context typedefs

Introduce Emacs_Pix_Container and Emacs_Pix_Context typedefs

Emacs_Pix_Container is a pointer to a struct representing pixmap data
on the backend.  Emacs_Pix_Context is the context for the
bitmap/pixmap on the backend.  Only w32 currently makes this
distinction; they are otherwise the same type.

* src/dispextern.h: Remove XImagePtr in favor of
using XImage* directly. Rename XImagePtr_or_DC to Emacs_Pix_Context.
[HAVE_X_WINDOWS] Alias Emacs_Pix_Container and
Emacs_Pix_Context to XImage*.
[HAVE_NS] Alias Emacs_Pix_Container and Emacs_Pix_Context to trivial
Emacs_Pixmap definition.
[HAVE_NTGUI]: Alias Emacs_Pix_Container to XImage* and
Emacs_Pix_Context to HDC.

* src/dispextern.h:
* src/image.c: Use Emacs_Pix_Container over XImagePtr and
Emacs_Pix_Context over XImagePtr_or_DC.