Moved declaration of format-specific image Load functions to dedicated header

The functions are declared in different files but they were called
in another file which re-declared the prototypes. This is dangerous
as it can lead to misaligned prototypes when functions changes.

They are now grouped in the library internal header 'imgformat.h'

wrlib/Makefile.am

@@ -19,6 +19,7 @@ bin_SCRIPTS = get-wraster-flags
 include_HEADERS = wraster.h
 
 libwraster_la_SOURCES = 	\
+	imgformat.h 	\
 	raster.c 	\
 	alpha_combine.c \
 	draw.c		\