Implement gradient filter of tight decoder, fixing issue #1767

This commit is a basic implementation of the gradient filter required by
qemu `lossy` option.

core/decoders/tight.js

@@ -285,7 +285,73 @@ export default class TightDecoder {
     }
 
     _gradientFilter(streamId, x, y, width, height, sock, display, depth) {
-        throw new Error("Gradient filter not implemented");
+        // assume the TPIXEL is 3 bytes long
+        const uncompressedSize = width * height * 3;
+        let data;
+
+        if (uncompressedSize === 0) {
+            return true;
+        }
+
+        if (uncompressedSize < 12) {
+            if (sock.rQwait("TIGHT", uncompressedSize)) {
+                return false;
+            }
+
+            data = sock.rQshiftBytes(uncompressedSize);
+        } else {
+            data = this._readData(sock);
+            if (data === null) {
+                return false;
+            }
+
+            this._zlibs[streamId].setInput(data);
+            data = this._zlibs[streamId].inflate(uncompressedSize);
+            this._zlibs[streamId].setInput(null);
+        }
+
+        let rgbx = new Uint8Array(4 * width * height);
+
+        let rgbxIndex = 0, dataIndex = 0;
+        let left = new Uint8Array(3);
+        for (let x = 0; x < width; x++) {
+            for (let c = 0; c < 3; c++) {
+                const prediction = left[c];
+                const value = data[dataIndex++] + prediction;
+                rgbx[rgbxIndex++] = value;
+                left[c] = value;
+            }
+            rgbx[rgbxIndex++] = 255;
+        }
+
+        let upperIndex = 0;
+        let upper = new Uint8Array(3),
+            upperleft = new Uint8Array(3);
+        for (let y = 1; y < height; y++) {
+            left.fill(0);
+            upperleft.fill(0);
+            for (let x = 0; x < width; x++) {
+                for (let c = 0; c < 3; c++) {
+                    upper[c] = rgbx[upperIndex++];
+                    let prediction = left[c] + upper[c] - upperleft[c];
+                    if (prediction < 0) {
+                        prediction = 0;
+                    } else if (prediction > 255) {
+                        prediction = 255;
+                    }
+                    const value = data[dataIndex++] + prediction;
+                    rgbx[rgbxIndex++] = value;
+                    upperleft[c] = upper[c];
+                    left[c] = value;
+                }
+                rgbx[rgbxIndex++] = 255;
+                upperIndex++;
+            }
+        }
+
+        display.blitImage(x, y, width, height, rgbx, 0, false);
+
+        return true;
     }
 
     _readData(sock) {