Add image parameters to webp exif data

[tijszwinkels]

As proposed by rockbandit here.

• exif metadata support for the .webp and .png file formats
• Now, we add exif data to the webp byte-stream as soon as it's downloaded from the server, even before it's written to browser localstorage. This way, exif data is always available, even when the user drag-and-drops the image from the webinterface or uses ->save as, as well as when using the happy path download button.

[tijszwinkels]

This is a follow-up to: https://github.com/daveschumaker/artbot-for-stable-diffusion/pull/147

tijs@Phoenix.local:~/os/artbot-for-stable-diffusion(webp_exif=)%$ exiftool  -usercomment -software ~/Downloads/saveas_test.webp
User Comment                    : A test setup in a futuristic universe, fractal, colorful.Negative prompt: bad quality, low quality, derpy.Steps: 20, Sampler: k_euler, CFG scale: 9, Seed: 3442820102, Size: 512x512, model: ICBINP - I Can't Believe It's Not Photography
Software                        : ArtBot - Create images with Stable Diffusion, utilizing the AI Horde
tijs@Phoenix.local:~/os/artbot-for-stable-diffusion(webp_exif=)%$ exiftool  -usercomment -software ~/Downloads/a_test_setup_in_a_futuristic_universe__fractal.jpg
User Comment                    : A test setup in a futuristic universe, fractal.Negative prompt: bad quality, low quality, derpy.Steps: 20, Sampler: k_euler, CFG scale: 9, Seed: 3774643812, Size: 512x512, model: ICBINP - I Can't Believe It's Not Photography
Software                        : ArtBot - Create images with Stable Diffusion, utilizing the AI Horde

works for both .webp saved through browser context menu, as well as 'normal' download button. :)