make image formats optional via features

[JAicewizard]

related to druid #1305, this PR makes piet default to no supported image formats, and allow adding support via features.
If only png and jpeg are required this cuts build times by 55% on my machine, and even with rayon build times are 35% less (for just piet).

added feature	total deps	total --realses build time (s)
0	3	1.3
image	21	4.9
image_png	32	5.5
jpeg	33	6.5
jpeg_rayon	60	9.7
gif	63	10.2
bmp	63	10.7
ico	63	10.8
tiff	65	12.2
webp	65	12.5
pnm	65	13
dds	65	13.7
tga	65	13.8
farbfeld	65	13.8
dxt	65	13.9
hdr	66	14.6

These build times are stacking, so the hdr build time also include png and jpeg...
But you can still clearly see that it is worth not having some of these, especially for the more obscure ones like everything after webp.
I have not benchmarked the impact on binary size, but I could make a small example program to see the impact on that.

[cmyr]

Okay, lets give this a shot. I was a little worried it would over-complicate things but we can always revisit if that ends up being the case.

[JAicewizard]

I don't like it either, but as it is the default features of image add a lot of bloat in total.
Please if you have another way of doing things linebender/druid#1305.
Should put in a PR for druid once this is in a pre-release?

[cmyr]

@JAicewizard sounds good!