Add F2 shortcut to take screenshots. Closes GH-119

[satoshinm]

https://github.com/satoshinm/NetCraft/issues/119

[satoshinm]

Works on the web version, but it opens a new window - can the app instead cause a file download, as to not interrupt gameplay? https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a documents the download attribute "If the attribute has a value, it is used as the pre-filled file name in the Save prompt (the user can still change the file name if they want).", can it be programmatically clicked?

[satoshinm]

<a download> click() works well, this was saved as screenshot-netcraft-Sun May 14 2017 16_48_04 GMT-0700 (PDT).png:

[satoshinm]

Now for native support. Could change to render to a texture, but as seen in https://github.com/satoshinm/NetCraft/issues/89 many platforms don't support drawBuffers() (hmm, but that's on the web, mobile?). http://stackoverflow.com/questions/5844858/how-to-take-screenshot-in-opengl suggests using glReadPixels() on the rendered frame instead.

[satoshinm]

miniz's png writer produces .. what looks like a png file:

native-build $ file screenshot-netcraft.png 
screenshot-netcraft.png: PNG image data, 2048 x 0, 8-bit/color RGBA, non-interlaced

but its 555 KB and I can't render it:

pngcrush tells what is wrong:

libpng warning: Image height is zero in IHDR
While converting NetCraft/native-build/screenshot-netcraft.png to /tmp/foo:
  pngcrush caught libpng error:
   Invalid IHDR data

   Critical chunk length, method 151 (ws 0 fm 0 zl 0 zs 0) =        0
   CPU time decoding 0.000, encoding 0.000, other 0.002, total 0.002 seconds

pngcheck is more specific - somehow it is 2048x0?!:

native-build $ pngcheck screenshot-netcraft.png 
screenshot-netcraft.png  invalid IHDR image dimensions (2048x0)
ERROR: screenshot-netcraft.png

supposed to be 2048x1536, that's g->width x g->height, where did it go wrong?

---

Oh this was already fixed upstream in miniz: https://github.com/richgel999/miniz/pull/65/files - it was a bug in miniz 2.0.5 beta.

[satoshinm]

Now only needs to have the filename timestamped on native (preferably, share date function from native and web, but if not no big deal), then this is ready to merge.

[satoshinm]