Introduce a non-generic Image base class

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

• Implements the proposal demonstrated in API proposal: Pixel-agnostic Image base class #897.
• Change majority of the extension methods working on Image<TPixel> to work on Image instead:

// The non-generic Image.Load overloads no longer return Image<Rgba32>
using (Image image = Image.Load("foo.jpg"))
{
    image.Mutate(x => x.Resize(100, 100);
    image.SaveAsPng(someStream);
}

• Touches Make ImageProcessor-s public #621 by making most of the non-generic processors public. (Exception: Histogram processors, because I'm not entirely happy with all namings.)
• Fix a few doc issues while doing the refactor.
• While implementing better test coverage for Image.Load variants, discovered and added a few missing overloads

Hope I haven't missed any of the important processors and / or extensions!

To enable extensions using color values as parameters on Image, we need to introduce a pixel-agnostic color type. I have a proposal for this, planning to PR it within the next week.

[antonfirsov]

Ok, went with the longer names. All processors without color parameters should be public (sealed) now.

[antonfirsov]

I'm merging this to go on with the rest of #907. If I made a mistake, I will be able to fix it in the follow up PR-s.