paintRect should support negative x and y

without this page, highlights which land outside of page in negative
direction will core-dump reader

blitbuffer.c

@@ -332,9 +332,25 @@ static int paintRect(lua_State *L) {
 	uint8_t *dstptr;
 
 	int cy;
-	if(w <= 0 || h <= 0 || x >= dst->w || y >= dst->h) {
-		return 0;
+
+	if(x < 0) {
+		if (x+w > 0) {
+			w += x;
+			x = 0;
+		} else {
+			return 0;
+		}
 	}
+
+	if(y < 0) {
+		if (y+h > 0) {
+			h += y;
+			y = 0;
+		} else {
+			return 0;
+		}
+	}
+
 	if(x + w > dst->w) {
 		w = dst->w - x;
 	}
@@ -342,6 +358,10 @@ static int paintRect(lua_State *L) {
 		h = dst->h - y;
 	}
 
+	if(w <= 0 || h <= 0 || x >= dst->w || y >= dst->h) {
+		return 0;
+	}
+
 	if(x & 1) {
 		/* This will render the leftmost column
 		 * in the case when x is odd. After this,