Support multi sources for images

Summary:
This adds support for specifying multiple sources for an image component, so that native can choose the best one based on the flexbox-computed size of the image.
The API is as follows: the image component receives in the `source` prop an array of objects of the type `{uri, width, height}`. On the native side, the native component will wait for the layout pass to receive the width and height of the image, and then parse the array to find the best fitting one. For now, this does not support local resources, but it will be added soon.
To see how this works and play with it, there's an example called `MultipleSourcesExample` under `ImageExample` In UIExplorer.

Reviewed By: foghina

Differential Revision: D3364550

fbshipit-source-id: 66c5aeb2794f2ffeff8da39a9c0b95155fb2d41f

Examples/UIExplorer/ImageExample.js

@@ -156,6 +156,63 @@ var ImageSizeExample = React.createClass({
   },
 });
 
+var MultipleSourcesExample = React.createClass({
+  getInitialState: function() {
+    return {
+      width: 30,
+      height: 30,
+    };
+  },
+  render: function() {
+    return (
+      <View style={styles.container}>
+        <View style={{flexDirection: 'row', justifyContent: 'space-between'}}>
+          <Text
+            style={styles.touchableText}
+            onPress={this.decreaseImageSize} >
+            Decrease image size
+          </Text>
+          <Text
+            style={styles.touchableText}
+            onPress={this.increaseImageSize} >
+            Increase image size
+          </Text>
+        </View>
+        <Text>Container image size: {this.state.width}x{this.state.height} </Text>
+        <View
+          style={[styles.imageContainer, {height: this.state.height, width: this.state.width}]} >
+          <Image
+            style={{flex: 1}}
+            source={[
+              {uri: 'http://facebook.github.io/react/img/logo_small.png', width: 38, height: 38},
+              {uri: 'http://facebook.github.io/react/img/logo_small_2x.png', width: 76, height: 76},
+              {uri: 'http://facebook.github.io/react/img/logo_og.png', width: 400, height: 400}
+            ]}
+          />
+        </View>
+      </View>
+    );
+  },
+  increaseImageSize: function() {
+    if (this.state.width >= 100) {
+      return;
+    }
+    this.setState({
+      width: this.state.width + 10,
+      height: this.state.height + 10,
+    });
+  },
+  decreaseImageSize: function() {
+    if (this.state.width <= 10) {
+      return;
+    }
+    this.setState({
+      width: this.state.width - 10,
+      height: this.state.height - 10,
+    });
+  },
+});
+
 exports.displayName = (undefined: ?string);
 exports.framework = 'React';
 exports.title = '<Image>';
@@ -510,6 +567,16 @@ exports.examples = [
       return <ImageSizeExample source={fullImage} />;
     },
   },
+  {
+    title: 'MultipleSourcesExample',
+    description:
+      'The `source` prop allows passing in an array of uris, so that native to choose which image ' +
+      'to diplay based on the size of the of the target image',
+    render: function() {
+      return <MultipleSourcesExample />;
+    },
+    platform: 'android',
+  },
 ];
 
 var fullImage = {uri: 'http://facebook.github.io/react/img/logo_og.png'};
@@ -567,4 +634,8 @@ var styles = StyleSheet.create({
     height: 50,
     resizeMode: 'contain',
   },
+  touchableText: {
+    fontWeight: '500',
+    color: 'blue',
+  },
 });