(XPutPixel): Handle monochrome images; used for masks.

(XPutPixel): Handle monochrome images; used for masks.
[HAVE_PNG]: Sync with xfns.c version.
(png_load): Adjust colors for Windows. Use Windows
bitmaps. Disable color table lookups.
(DEF_IMGLIB_FN, LOAD_IMGLIB_FN): New macros.
(init_png_functions): New function.
(png_read_from_memory, png_load): Call png library functions
through pointers determined at runtime.
(QCloader, QCbounding_box, QCpt_width, QCpt_height): Declare.
(init_external_image_libraries): New function.
(init_xfns): Call it.