jpeg-handler: Fix issue with libjpeg62.

mandelbrot-example: Create 3 channel image instead of 4 channel image
during jpg creation. The previous version can not be handled by all
variants of libjpeg.

examples/mandelbrot/mandelbrot.cpp

@@ -33,8 +33,8 @@
 #include <onion/extras/jpeg.h>
 #endif
 
-#define MAX(X,Y) (((X)>(Y))?X:Y)
-#define MIN(X,Y) (((X)<(Y))?X:Y)
+#define MAX(X,Y) (((X)>(Y))?(X):(Y))
+#define MIN(X,Y) (((X)<(Y))?(X):(Y))
 
 /// Basic complex class, to ease the fractal calculation
 class Complex{
@@ -133,7 +133,7 @@ int mandelbrotJPEG(void *p, onion_request *req, onion_response *res){
 
 	int channels = 1;
 	if( color == 1)
-		channels = 4;
+		channels = 3;
 
 	unsigned char *image=new unsigned char[channels*width*height];
   int    i,j,n;
@@ -161,19 +161,18 @@ int mandelbrotJPEG(void *p, onion_request *req, onion_response *res){
 			if (n >= steps) P=255;
 			else P=(n*256)/steps;
 
-			if( channels == 4 ){
+			if( channels == 3 ){
 				*imagep++=P;
 				*imagep++=2*P+30;
 				*imagep++=255-P*3;
-				*imagep++=255;
 			}else{
 				*imagep++=P;
 			}
     }
   }
 
 	onion_jpeg_response(image, channels,
-			(channels==1?JCS_GRAYSCALE:JCS_EXT_RGBX)
+			(channels==1?JCS_GRAYSCALE:JCS_RGB)
 			, width, height, jpgQuality, res);
 
 	delete[] image;

src/onion/extras/jpeg.c

@@ -86,7 +86,13 @@ void onion_init_destination(j_compress_ptr cinfo)
 boolean onion_empty_output_buffer(j_compress_ptr cinfo)
 {
 	onion_jpeg_data *d=(onion_jpeg_data*)cinfo->client_data;
+	/*
+	 * Note: It's not correct to substract free_in_buffer. This value
+	 * should be zero, but this depends on the implementation of libjpeg.
+	 * It's better to assume that all bytes of the buffer are used.
 	d->res->buffer_pos = sizeof(d->res->buffer)-cinfo->dest->free_in_buffer;
+	 */
+	d->res->buffer_pos = sizeof(d->res->buffer);
 
 	if(onion_response_flush(d->res)<0){
 		// Flush failed.