Add full support for ragg

[thomasp85]

Fix #4347

This PR does two things: First, it will use ragg as the default device if it is installed when generating png, jpeg, and tiff files. Second, it fixes the issue that has crippled the use of device = agg_png in ggsave, in that it explicitly sets the resolution and units to use in the device function that is passed in. While this only works for devices that uses res and units arguments it means that devices that uses the same signature as png() will work the same way

[clauswilke]

While you're at it, I would suggest to add at least the unit px, and maybe pt also. It seems strange to me that we can't specify output in px, the most commonly used unit in web design.

[thomasp85]

Yeah — px should at least be supported. I don't think pt has much merit as an output dimension unit?

[clauswilke]

Normally my attitude would be "it doesn't cost much to add pt so why not" but I can see that people might get confused about the difference between px and pt so maybe better leave it out.

[thomasp85]

Yeah, that is kinda my worry as well

[thomasp85]

I've added px support - @clauswilke can you give this a review?