Adds the ability to capture opaque CGImages from GPUImageFilters

[RobRuana]

GPUImage is a fantastic library, thanks for all the hard work!

I frequently use GPUImage to create images for use in a UITableView or UICollectionView (creating thumbnails, adding a drop shadow, etc...).

One way to achieve smooth scrolling with a UITableView is to ensure that every image in the table is fully opaque, with no alpha channel. A quick google for "UITableView performance" yields plenty of pages that describe why opaque images are important.

I often find myself writing code that does the following:

Source UIImage
    |
    +-> GPUImageFilter
        |
        +-> Output UIImage
            |
            +-> CoreGraphics removeAlpha()
                |
                +-> Opaque UIImage

using a UIImage extension like this:

extension UIImage {
    func removeAlpha() -> UIImage {
        UIGraphicsBeginImageContextWithOptions(size, true, scale)
        self.drawInRect(CGRectMake(0, 0, size.width, size.height))
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }
}

Which is just crazy, because I'm needlessly using the CPU to create a copy of an image that I just created on the GPU. It would be nice if I could just ask GPUImage to make the UIImage opaque when it's initially created.

This pull request adds a boolean flag shouldCaptureOpaqueImage to GPUImageOutput and GPUImageFrameBuffer to enable this behavior.

Thanks again for creating GPUImage!