Add tint color for iOS

The tint color can be used with a texture to tint the texture, or alone
to set a flat color for the model if no texture property is specified.
The tint property has to be a javascript object of the following format:
{r: 1.0, g: 1.0, b: 1.0, a: 1.0}.

index.js

@@ -14,6 +14,7 @@ GLModelView.propTypes = {
 
   model: PropTypes.string.isRequired,
   texture: PropTypes.string,
+  tint: PropTypes.object,
 
   rotateX: PropTypes.number,
   rotateY: PropTypes.number,

ios/RNGLModelView.m

@@ -16,7 +16,7 @@ - (void)didMoveToWindow
   [super didMoveToWindow];
 
   // Setup GLViewModel
-  self.blendColor = nil;
+  //self.blendColor = [UIColor colorWithRed:1.0 green:0.0 blue:0.0 alpha:1.0];
 
   if (self.window) {
     // Render with our applied props!

ios/RNGLModelViewManager.m

@@ -52,6 +52,19 @@ - (UIView *)view
   view.texture = [GLImage imageNamed:[RCTConvert NSString:json]];
 }
 
+RCT_CUSTOM_VIEW_PROPERTY(tint, NSDictionary, RNGLModelView)
+{
+  NSDictionary *tint = [RCTConvert NSDictionary:json];
+
+  // If the alpha is not specified, we assume that the user wants a fully opaque object
+  float alpha = [tint objectForKey:@"a"] == nil ? 1.0 : [[tint valueForKey:@"a"] floatValue];
+  float red = [[tint valueForKey:@"r"] floatValue];
+  float green = [[tint valueForKey:@"g"] floatValue];
+  float blue = [[tint valueForKey:@"b"] floatValue];
+
+  view.blendColor = [UIColor colorWithRed:red green:green blue:blue alpha:alpha];
+}
+
 // Starts or stops the animation loop to automatically re-render each 1/60 second
 RCT_CUSTOM_VIEW_PROPERTY(animate, BOOL, RNGLModelView)
 {