Fill padding color of cropped image with background color of view

[daisy1754]

Currently, if users scale an image smaller than bounds area, a padding area is filled with black color. This PR change the behaviour to filling padding with a color of the background view. The result should be more WYSIWYG for users.

[daisy1754]

I made another commit to answer discussion above. Please have a look 😄

[adahan]

Hi, I tried to replace "blackColor" by "clearColor". This doesn't change anything. Someone could give me a hint about how I can get a clear background? Thanks in advance

Edit: I found the answer. My source hadn't any alpha chanel.
=> Solution to add alpha canal to add in -(CGIImageRef)newTransformedImage method

//ADD ALPHA CHANEL
{
CGImageAlphaInfo alpha = CGImageGetAlphaInfo(source);
BOOL hasAlpha = (alpha == kCGImageAlphaFirst || alpha == kCGImageAlphaLast || alpha == kCGImageAlphaPremultipliedFirst || alpha == kCGImageAlphaPremultipliedLast);

if (!hasAlpha) {
    size_t width = CGImageGetWidth(source);
    size_t height = CGImageGetHeight(source);

    CGContextRef offscreenContext = CGBitmapContextCreate(NULL,
                                                          width,
                                                          height,
                                                          CGImageGetBitsPerComponent(source),
                                                          0,
                                                          CGImageGetColorSpace(source),
                                                          kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);

    CGContextDrawImage(offscreenContext, CGRectMake(0, 0, width, height), source); 
    source = CGBitmapContextCreateImage(offscreenContext);

    CGContextRelease(offscreenContext);
}

//ADD ALPHA CHANEL