Add config so UIImages fit the image automatically

[nchamo]

Right now, when a UIImage is used on the SDK, content creators need to specify the sourceWidth and sourceHeight. This values represent the size of the original image. Then, the content creator will need to specify width and height, which represent the final size on screen.

Having to specify sourceWidth and sourceHeight is truly uncomfortable, specially since most platforms already fit the image automatically. There are also some cases, with images used from other servers, where the size is unknown.

The idea would be to support this on the SDK, by adding a boolean flag (maybe called something like fitImage). If the boolean were to be true, then the renderer would ignore the sourceWidth and sourceHeight values, and simply fit it completely.