Implement background blur on Safari.

Safari doesn't implement filtered drawing on HTML canvas,
it silently ignores the "filter" property of Context2D.
We implement our own blurring by scaling the background down
and then back up.

static/galene.js

@@ -1341,13 +1341,6 @@ let filters = {
     'background-blur': {
         description: 'Background blur',
         predicate: async function() {
-            if(isSafari()) {
-                console.warn(
-                    'Background blur does not work on Safari, disabled.'
-                );
-                return false;
-
-            }
             let r = await fetch('/third-party/tasks-vision/vision_bundle.mjs', {
                 method: 'HEAD',
             });
@@ -1399,21 +1392,48 @@ let filters = {
 
                 // set the alpha mask, background is opaque
                 ctx.globalCompositeOperation = 'copy';
-                ctx.filter = 'none';
                 ctx.drawImage(mask, 0, 0);
 
                 // rather than blurring the original image, we first mask
                 // the background then blur, this avoids a halo effect
                 ctx.globalCompositeOperation = 'source-in';
-                ctx.filter = 'none';
                 ctx.drawImage(result.bitmap, 0, 0);
+		if('filter' in ctx) {
                     ctx.globalCompositeOperation = 'copy';
                     ctx.filter = `blur(${src.videoWidth / 48}px)`;
                     ctx.drawImage(ctx.canvas, 0, 0);
+                    ctx.filter = 'none';
+		} else {
+		    // Safari bug 198416, context.filter is not supported.
+		    let scale = 24;
+		    let swidth = src.videoWidth / scale;
+		    let sheight = src.videoHeight / scale;
+		    if(!('canvas' in this.userdata))
+			this.userdata.canvas = document.createElement('canvas');
+		    let c2 = this.userdata.canvas;
+		    if(c2.width != swidth)
+			c2.width = swidth;
+		    if(c2.height != sheight)
+			c2.height = sheight;
+		    let ctx2 = c2.getContext('2d');
+		    // scale down the background
+		    ctx2.globalCompositeOperation = 'copy';
+		    ctx2.drawImage(ctx.canvas,
+				   0, 0, src.videoWidth, src.videoHeight,
+				   0, 0, swidth, sheight,
+				  );
+		    // scale back up, composite atop the original background
+		    ctx.globalCompositeOperation = 'source-atop';
+		    ctx.drawImage(ctx2.canvas,
+				  0, 0,
+				  src.videoWidth / scale,
+				  src.videoHeight / scale,
+				  0, 0, src.videoWidth, src.videoHeight,
+				 );
+		}
 
 		// now draw the foreground
                 ctx.globalCompositeOperation = 'destination-atop';
-                ctx.filter = 'none';
                 ctx.drawImage(result.bitmap, 0, 0);
                 ctx.globalCompositeOperation = 'source-over';