image alt attr in tile imagery and tag wiki info

modules/renderer/tile_layer.js

@@ -198,6 +198,7 @@ export function rendererTileLayer(context) {
         image.enter()
           .append('img')
             .attr('class', 'tile')
+            .attr('alt', '')
             .attr('draggable', 'false')
             .style('width', _tileSize + 'px')
             .style('height', _tileSize + 'px')

modules/ui/tag_reference.js

@@ -53,6 +53,7 @@ export function uiTagReference(what) {
             _body
                 .append('img')
                 .attr('class', 'tag-reference-wiki-image')
+                .attr('alt', docs.description)
                 .attr('src', docs.imageURL)
                 .on('load', function() { done(); })
                 .on('error', function() { d3_select(this).remove(); done(); });