Add RenderQRCode example

[antonfirsov]

This example:

• Demonstrates how to process an image row-by row with a real-life use-case
• Demonstrates the usage of a non-default L8 pixel type, and saving the result to a GrayScale 1bpp PNG
• May help users of the popular QRCoder library to mitigate the obsolation of System.Drawing in their own applications. (byte[,] pattern should be compatible with AbstractQRCode.QrCodeData.ModuleMatrix)
• May help QRCoder to port the library to ImageSharp -- see System.Drawing.Common is deprecated on non-Windows platforms codebude/QRCoder#315

/cc @JimBobSquarePants @safern

[tocsoft]

I would suggest if the example is called "Render QR Code" we should probably just add a reference to a QR code library instead of hard coding some bitmap, otherwise its not really QR code rendering it just an example for twiddling pixels

Its not a useful sample for end users unless its actually end to end, most users won't be able to take this sample and actually wire it into QR code gen.

I feel this should really be split into 2 examples, 1 with a QRCoder dependency and one using ZXing.Net.

[antonfirsov]

@tocsoft this is a reasonable suggestion, however in case of QRCoder I would end up implementing codebude/QRCoder#315 myself. It might be trivial job, but for case there are some unknown quirks, I'm unsure if it's the best investment of my time ATM. By providing a pixel twiddling example for now, we will help the library owners to solve the issue, and may also help some users to adapt a workaround, or just show how to manipulate pixels with less trivial example.