factored out blitting constraints check in a seperate function

the checking of the boundaries when blitting was incomplete
for the addblitToBuffer() function when compared with the
blitToBuffer() function. Factored out the redundant checking
procedure into a new function.

Should fix another bug leading to issue #401 and probably
more cases for specifying boundary boxes.

blitbuffer.c

@@ -105,69 +105,80 @@ static int blitFullToBuffer(lua_State *L) {
 	return 0;
 }
 
-static int blitToBuffer(lua_State *L) {
-	BlitBuffer *dst = (BlitBuffer*) luaL_checkudata(L, 1, "blitbuffer");
-	BlitBuffer *src = (BlitBuffer*) luaL_checkudata(L, 2, "blitbuffer");
-	int xdest = luaL_checkint(L, 3);
-	int ydest = luaL_checkint(L, 4);
-	int xoffs = luaL_checkint(L, 5);
-	int yoffs = luaL_checkint(L, 6);
-	int w = luaL_checkint(L, 7);
-	int h = luaL_checkint(L, 8);
-	int x, y;
-
+/**
+* check/adapt boundaries for blitting operations
+*
+* @return 0 if no blitting is needed, 1 otherwise
+*/
+int fitBlitBufferBoundaries(BlitBuffer* src, BlitBuffer* dst, int* xdest, int* ydest, int* xoffs, int* yoffs, int* w, int* h) {
 	// check bounds
-	if(ydest < 0) {
+	if(*ydest < 0) {
 		// negative ydest, try to compensate
-		if(ydest + h > 0) {
+		if(*ydest + *h > 0) {
 			// shrink h by negative dest offset
-			h += ydest;
+			*h += *ydest;
 			// extend source offset
-			yoffs += -ydest;
-			ydest = 0;
+			*yoffs += -(*ydest);
+			*ydest = 0;
 		} else {
 			// effectively no height
 			return 0;
 		}
-	} else if(ydest >= dst->h) {
+	} else if(*ydest >= dst->h) {
 		// we're told to paint to off-bound target coords
 		return 0;
 	}
-	if(ydest + h > dst->h) {
+	if(*ydest + *h > dst->h) {
 		// clamp height if too large for target size
-		h = dst->h - ydest;
+		*h = dst->h - *ydest;
 	}
-	if(yoffs >= src->h) {
+	if(*yoffs >= src->h) {
 		// recalculated source offset is out of bounds
 		return 0;
-	} else if(yoffs + h > src->h) {
+	} else if(*yoffs + *h > src->h) {
 		// clamp height if too large for source size
-		h = src->h - yoffs;
+		*h = src->h - *yoffs;
 	}
 	// same stuff for x coords:
-	if(xdest < 0) {
-		if(xdest + w > 0) {
-			w += xdest;
-			xoffs += -xdest;
-			xdest = 0;
+	if(*xdest < 0) {
+		if(*xdest + *w > 0) {
+			*w += *xdest;
+			*xoffs += -(*xdest);
+			*xdest = 0;
 		} else {
 			return 0;
 		}
-	} else if(xdest >= dst->w) {
+	} else if(*xdest >= dst->w) {
 		return 0;
 	}
-	if(xdest + w > dst->w) {
-		w = dst->w - xdest;
+	if(*xdest + *w > dst->w) {
+		*w = dst->w - *xdest;
 	}
-	if(xoffs >= src->w) {
+	if(*xoffs >= src->w) {
 		return 0;
-	} else if(xoffs + w > src->w) {
-		w = src->w - xoffs;
+	} else if(*xoffs + *w > src->w) {
+		*w = src->w - *xoffs;
 	}
+	return 1; // continue processing
+}
+
+static int blitToBuffer(lua_State *L) {
+	BlitBuffer *dst = (BlitBuffer*) luaL_checkudata(L, 1, "blitbuffer");
+	BlitBuffer *src = (BlitBuffer*) luaL_checkudata(L, 2, "blitbuffer");
+	int xdest = luaL_checkint(L, 3);
+	int ydest = luaL_checkint(L, 4);
+	int xoffs = luaL_checkint(L, 5);
+	int yoffs = luaL_checkint(L, 6);
+	int w = luaL_checkint(L, 7);
+	int h = luaL_checkint(L, 8);
+	int x, y;
 
 	uint8_t *dstptr;
 	uint8_t *srcptr;
 
+	if(!fitBlitBufferBoundaries(src, dst, &xdest, &ydest, &xoffs, &yoffs, &w, &h))
+		return 0;
+
 	if(xdest & 1) {
 		/* this will render the leftmost column */
 		dstptr = (uint8_t*)(dst->data +
@@ -248,31 +259,12 @@ static int addblitToBuffer(lua_State *L) {
 	int h = luaL_checkint(L, 8);
 	int x, y;
 
-	// check bounds
-	if(yoffs >= src->h) {
-		return 0;
-	} else if(yoffs + h > src->h) {
-		h = src->h - yoffs;
-	}
-	if(ydest >= dst->h) {
-		return 0;
-	} else if(ydest + h > dst->h) {
-		h = dst->h - ydest;
-	}
-	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;
 
+	if(!fitBlitBufferBoundaries(src, dst, &xdest, &ydest, &xoffs, &yoffs, &w, &h))
+		return 0;
+
 	if(xdest & 1) {
 		/* this will render the leftmost column */
 		dstptr = (uint8_t*)(dst->data +