Disable simulcast for screen sharing.

Simulcast doesn't seem to work well with screen sharing: only one
layer is sent, which has very low throughput since we send the low
layer first.  Disable simulcast for screen sharing.

static/galene.html

@@ -230,9 +230,8 @@
             <select id="requestselect" class="select select-inline">
               <option value="">nothing</option>
               <option value="audio">audio only</option>
-              <option value="screenshare-low">screen share (low)</option>
-              <option value="screenshare">screen share</option>
-              <option value="everything-low">everything (low)</option>
+              <option value="screenshare">screenshare only</option>
+              <option value="everything-low">low quality</option>
               <option value="everything" selected>everything</option>
             </select>
           </form>

static/galene.js

@@ -607,9 +607,6 @@ function mapRequest(what) {
     case 'audio':
         return {'': ['audio']};
         break;
-    case 'screenshare-low':
-        return {screenshare: ['audio','video-low'], '': ['audio']};
-        break;
     case 'screenshare':
         return {screenshare: ['audio','video'], '': ['audio']};
         break;
@@ -1227,7 +1224,7 @@ function setUpStream(c, stream) {
             let bps = getMaxVideoThroughput();
             // Firefox doesn't like us setting the RID if we're not
             // simulcasting.
-            if(simulcast) {
+            if(simulcast && c.label !== 'screenshare') {
                 encodings.push({
                     rid: 'h',
                     maxBitrate: bps || unlimitedRate,