rn,flags: add ability to override resolution using a flag

Also, use the configured resolution to set it as the max received frame size.

react/features/base/conference/middleware.js

@@ -460,7 +460,10 @@ function _sendTones({ getState }, next, action) {
  */
 function _setReceiverVideoConstraint(conference, preferred, max) {
     if (conference) {
-        conference.setReceiverVideoConstraint(Math.min(preferred, max));
+        const value = Math.min(preferred, max);
+
+        conference.setReceiverVideoConstraint(value);
+        logger.info(`setReceiverVideoConstraint: ${value}`);
     }
 }
 

react/features/base/config/middleware.js

@@ -3,6 +3,7 @@
 import { jitsiLocalStorage } from 'js-utils';
 
 import { APP_WILL_MOUNT } from '../app';
+import { getFeatureFlag } from '../flags/functions';
 import { addKnownDomains } from '../known-domains';
 import { MiddlewareRegistry } from '../redux';
 import { parseURIString } from '../util';
@@ -107,6 +108,12 @@ function _setConfig({ dispatch, getState }, next, action) {
         config.p2p = { enabled: !settings.disableP2P };
     }
 
+    const resolutionFlag = getFeatureFlag(state, 'resolution');
+
+    if (typeof resolutionFlag !== 'undefined') {
+        config.resolution = resolutionFlag;
+    }
+
     dispatch({
         type: _UPDATE_CONFIG,
         config

react/features/base/flags/constants.js

@@ -81,6 +81,13 @@ export const RAISE_HAND_ENABLED = 'raise-hand.enabled';
  */
 export const RECORDING_ENABLED = 'recording.enabled';
 
+/**
+ * Flag indicating the local and (maximum) remote video resolution. Overrides
+ * the server configuration.
+ * Default: (unset).
+ */
+export const RESOLUTION = 'resolution';
+
 /**
  * Flag indicating if server URL change is enabled.
  * Default: enabled (true)