A take at better boundary checks in blitbuffer

this will hopefully prevent segfaults resulting from bad
coordinate/size input when blitting

blitbuffer.c

@@ -93,26 +93,53 @@ static int blitToBuffer(lua_State *L) {
 	int x, y;
 
 	// check bounds
+	if(ydest < 0) {
+		// negative ydest, try to compensate
+		if(ydest + h > 0) {
+			// shrink h by negative dest offset
+			h += ydest;
+			// extend source offset
+			yoffs += -ydest;
+			ydest = 0;
+		} else {
+			// effectively no height
+			return 0;
+		}
+	} else if(ydest >= dst->h) {
+		// we're told to paint to off-bound target coords
+		return 0;
+	}
+	if(ydest + h > dst->h) {
+		// clamp height if too large for target size
+		h = dst->h - ydest;
+	}
 	if(yoffs >= src->h) {
+		// recalculated source offset is out of bounds
 		return 0;
 	} else if(yoffs + h > src->h) {
+		// clamp height if too large for source size
 		h = src->h - yoffs;
 	}
-	if(ydest >= dst->h) {
+	// same stuff for x coords:
+	if(xdest < 0) {
+		if(xdest + w > 0) {
+			w += xdest;
+			xoffs += -xdest;
+			xdest = 0;
+		} else {
+			return 0;
+		}
+	} else if(xdest >= dst->w) {
 		return 0;
-	} else if(ydest + h > dst->h) {
-		h = dst->h - ydest;
+	}
+	if(xdest + w > dst->w) {
+		w = dst->w - xdest;
 	}
 	if(xoffs >= src->w) {
 		return 0;
 	} else if(xoffs + w > src->w) {
 		w = src->w - xoffs;
 	}
-	if(xdest >= dst->w) {
-		return 0;
-	} else if(xdest + w > dst->w) {
-		w = dst->w - xdest;
-	}
 
 	uint8_t *dstptr;
 	uint8_t *srcptr;