add libXfont2 support with a compile-time switch

[stefan11111]

Same as #12, but with a compile time switch between libXfont1 and libXfont2.

Defaults to libXfont1

[stefan11111]

while doing this, I also found that there is a function 'miImageGlyphBlt' in mi/miglblt.c, which is declared static(_X_EXPORT), only referenced in mi/mi.h, and never called anywhere.

[rofl0r]

since all xfont2 specific functions seem to behave identically, except for the name, i'd add a xfont2_compat.h which looks like

#ifdef XFONT2
#define AllocateFontPrivateIndex xfont2_allocate_font_private_index
...
#endif

that way the ifdefs are in a single place and not littered all over the code.
(one could also use function-like macros)

[stefan11111]

Implemented @rofl0r 's suggestion.
There are still a few ifdef's in various source files, but they are no longer just symbols missing in linXfont2 that are there under a different name.

Will open a new PR with everything squashed in one single commit.

[stefan11111]

Opened the new PR here: #14