move png support to separate file

Makefile

@@ -1174,6 +1174,7 @@ Q3OBJ = \
   $(B)/client/tr_flares.o \
   $(B)/client/tr_font.o \
   $(B)/client/tr_image.o \
+  $(B)/client/tr_image_png.o \
   $(B)/client/tr_init.o \
   $(B)/client/tr_light.o \
   $(B)/client/tr_main.o \

code/renderer/tr_image_png.h

@@ -0,0 +1 @@
+void LoadPNG( const char *name, byte **pic, int *width, int *height );