Change bitmap signedness to allow conversion to other data types

Buffered bitmap array should have been type unsigned char not type char
Includes change to manual
In response to (and hopefully fixing) #182 reported by Marcelo Antunes

backend/zint.h

@@ -125,7 +125,7 @@ extern "C" {
         unsigned char encoded_data[200][143];
         int row_height[200]; /* Largest symbol is 189 x 189 Han Xin */
         char errtxt[100];
-        char *bitmap;
+        unsigned char *bitmap;
         int bitmap_width;
         int bitmap_height;
         unsigned int bitmap_byte_length;