change struct NSVGpaint:type to signed char

Commit 34a6c49 added NSVG_PAINT_UNDEF as -1. For this to work correctly
where char is unsigned by default, struct NSVGpaint::type needs changing
to explicit signed char. (An 'int' would work too, but I did not want to
change the size of the stucture.)

src/nanosvg.h

@@ -120,7 +120,7 @@ typedef struct NSVGgradient {
 } NSVGgradient;
 
 typedef struct NSVGpaint {
-	char type;
+	signed char type;
 	union {
 		unsigned int color;
 		NSVGgradient* gradient;